Tuesday, June 16, 2015

Substitue Battery Charger for Generac Generator

Generator Starter Battery Charger Problems 

My wife's parents have a 15kW Generac generator that has a problem keeping the starting battery charged. After charging the battery the weekly auto-start would work for a month or two and then fail to start. The unit has been serviced and tested and the battery replaced without resolving the issue. I wanted to see if I could figure out how to fix the problem.

A quick Google search reveals that battery charger issues seem to be common with Generac generators (not to pick on Generac, this may be a common issue).  I found a system schematic in the owner's manual and began studying the battery charger wiring.

Generac Owner's Manual

It appears there are two configurations, both using a dedicated battery charging module. One configuration receives power via a pair of contactors that selects power from either a transformer when utility power is available or a dedicated BATTERY CHARGE WINDING when the generator is running. The other configuration has the battery charger labeled (UTILITY) which appears to indicate that the charger runs only from utility power and not while the generator is running

Why would the generator not run the battery charger from the generator's output rather than adding a separate winding and a set of contactors? I found several posts that mention that the Generac battery charger does not charge while the generator is running, so utility only charging appears to be common.

Replacement battery charger modules are available at prices starting round $120. Below is a link and photo of  a charger module similar to the one in generator I was working on.

Replacement Generac battery charger module

It is not my intent to cast stones at another engineer's design, but the module looks a little kludgy with the mix of internal and external parts, most encapsulated but several not. I do understand that the orange overcurrent protection devices must be able to expand to function properly.

Given the number of complaints I found online about Generac battery charging, I was reluctant to recommend another service call to replace the OEM charger module.

I suspect the core issue is that designing battery chargers is not Generac's primary focus so the battery charger was not seen as a high engineering priority. Perhaps a battery charger designed by people who are in the business of designing and selling battery chargers might be more reliable in keeping the generator starter battery topped off and ready to go.

Charger Options. 

I wanted a charger with  a few specific features:
 - designed to be left connected to the battery
 - waterproof for outdoor mounting
 - can be powered by the generator
 - good brand reputation

There are several chargers on the market designed for continuous charging of the battery during storage but the list narrows quickly once you focus on chargers that are designed for outdoor mounting. I ended up selecting the Battery Tender 800.

Battery Tender 800, $40 at Amazon

This unit is epoxy encapsulated with an  IP67 rating (which means the unit is dust tight and can tolerate submersion in water to 1 meter). It is also highly rated on Amazon (most complaints were that warranty replacement requires $10 for processing and you pay shipping).

The ability to operate on 240 VAC was a plus simply because it let me use convenient screw terminals for power attachment in the generator. Using a 120 VAC powered unit would require a large ring terminal to fit the larger Neutral terminal. The Battery Tender 800 has two mounting holes and comes with mounting screws (not used here but handy if you want to mount it in a convenient spot near your motorcycle, boat, lawnmower, et cetera). And at this price you can replace it 3 times before reaching the price of the OEM charger module.

Battery Tender 800 Owner's Manual

Danger, Will Robinson!

There are so many dangerous parts to a generator that it is hard to know where to start with warnings. You can get burned, mangled or just plain dead. You should not work on your generator unless you are confident you know what you are doing. Bet your life on it confident. Even then, it's not a good idea. Have a generator service technician install the battery charger for you. If you are so foolish as to proceed, here is what I did.

Note that while I am an electrical engineer, I have little relevant experience with 240 VAC and generators and you should not think for a minute that I know what I am doing. I know that there must be a more elegant way to accomplish the task at hand. My overriding goal was not efficiency but rather not killing myself or blowing up something.

First I went through the house shutting down sensitive electronics like the PC and TV's since I was going to turn off the power. Then I went to the generator and turned off the generator's breaker. Then I used the transfer switch to disconnect the utility power (the generator and house were now without power). Then I removed the top panel from the generator electronics board and removed the front panel of the generator to provide access to the battery bay.

240 VAC is available on terminals N1 and N1 near the bottom center of the photo. Note notch in top left of photo where I will route the power cable. The large terminal with the cable wrapped in white appears to be the NEUTRAL terminal.

Then I checked the battery voltage with my multi-meter and install the battery charger on the battery using the ring terminals. I placed the charger on top of the battery positioned so I could see the status LED.

The 240 VAC input  was available on screw terminals labeled N1 and N2. Next I cut the 120 VAC plug from the cord and crimped terminals on the wires. I could have used 120 VAC by connecting one terminal to N1 or N2 and the other to the NEUTRAL terminal (it looks like a large ring terminal would be needed). I could also have wired a short extension cord for 120 VAC and not removed the plug from the charger in case I needed to have it replaced under warranty. I will try that approach if I have to replace this charger.

Then I ran the power input lead to the back of the battery bay and through a vent at the left rear of bay. This brought the cable into the rightmost side of the engine bay where it can be routed up and into the back left corner of the electronics bay. There is a notch that allowed the electronics bay cover to be replaced without conflicting with the wire. Here is a blurry photo (how did I not see how blurry this was when I was taking the pictures).

I ran the charger cable to the back of the battery bay and through a small vent to the engine compartment and then up through a notch in the sheet metal to reach the electronics control area. It's the lamp cord looking cable just beneath the hinge.

I wanted to connect the battery charger power lead to terminals N1 and N2. First I carefully used my multi-meter and verified that there is no voltage on the terminals. Next I pretended the terminals still had lethal voltage levels and carefully placed one terminal at a time on the screw terminals (this meant two spade terminals on the terminal so I went slowly and made sure everything looked clean).

Charger terminals installed on N1 and N2.
Once the terminals were snug, I reversed the process, putting the cover back over the electronics bay (I kept the breaker off for the moment) and used the transfer switch to restore power to the house. Now I turned on the breaker on the generator and checked the status LED on the charger. The LED indicated the battery was being charged (yellow) but could have been green if the battery was already fully charged. I measured the battery voltage and verified that the voltage was increasing (if the LED indicates the battery is charging). Then I used the manual switch to start the generator.

The generator started without a problem. Finally I turned off the generator, placed the red safety cover back over the battery positive terminal and replaced the front panel on the generator. The charger has been working well for 6 months.

As of March, 2017, the generator battery charger has been working well for almost two years.


The manufacturer has updated the design of the selected charger (for higher efficiency and 4-stage charging) and the input voltage range was reduced to 100 - 130 VAC. The new model has a red LED to indicate charging status and has a red and green battery icon to the left of the LED. The new models retain their awesome IP67 rating. Always verify the voltage range marked on the charger!

The reduced operating voltage range means you must simply have your electrician or generator service technician connect one terminal of the charger's power cable to EITHER N1 or N2 and the other to the large NEUTRAL terminal (bolt with the cable wrapped in white tape).
New design with red LED and 100-130 VAC input voltage.

Older design with a yellow LED and yellow and green battery
 icon to the left of the LED  and 100-240 VAC input voltage.


  1. Pretty article! I found some useful information in your blog, it was awesome to read, thanks for sharing this great content to my vision, keep sharing.
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    1. Thanks! Glad you enjoyed it!


    2. Shane, thank you very much for your post/instructions! My 12 years with my Generac Guardian 15 KW/LPG have been very eventful.
      That said, a home generator is one of a few best investments that I ever made.
      The very first battery, a Sears Diehard,over charged and spew acid throughout. I'm still dealing with the rust, because of it.

      2 1/2 years ago, the generator determined a power outage and started. The problem was that the whole house transfer switch didn't engage. $2000, a new main board and transfer switch rebuild by 3 "Generac Approved" service companies in N.E. PA the
      result was the same. I don't want to bore all with the weeds, but I finally found "The Wire Guys" in NEPA who diagnosed the problem as a broken under ground wire (3 feet). It probably happened when I had my driveway paved. I couldn't get a new "control" wire strand through the conduit. Lesson: Use 3 X larger
      than you need, at the time. The wire guys had to do, with my assistance, a rewire above ground, through walls etc.

      Now I did follow your post, almost, to the "T."
      Very easy, just shut down generator first and main cut off
      to house. I connected a 2.5 foot 3 wire lead with a 3 prong plug, via the rubber LPG engine gasline port, from battery compartment to the engine. I connected the black/hot wire to N2(easier access than N1 on my unit),the neutral white to the respective bolt and same for the Green ground wire and bolt. Be aware,for polarity,that the neutral white wire connects to the taller or "wider" male/female plug. I remember by "white equals wide"
      N2 is 120 V each, but only 5 watts. That may explain why it took 2 days to fully charge, red to green, the battery. The charger only has 2 wires, but I still added the ground. Aside from the 3 year warranty on the "Battery Tender 800, shipping costs may exceed (by design) just buying a new one of any make.
      The biggest advantage is now, for me a "plug and play." I don't have to shut the house down to rewire. I also stuck the charger to inner wall with Velcro. I can see the charger light, now green by opening the lid at dark, but would be nice to see another way. I just haven't figured out how yet.

    3. Steven,

      I am glad you liked the post. Yes, the Battery Tender will take a while to recharge a flat battery. I like the idea of mounting the charger where you can see the LED. If you have enough cord you can mount it outside the chassis since the Battery Tender 800 interior is encapsulated, making it weather proof.


    4. At this point, I can, and do pop the Guardian Lid, once a week and look for the green LED light on the charger. To be honest, I've had far better luck with Schumacher chargers, over the years than with the Battery Tender brand. Currently, I have 4 of the latter that don't work, but all of my Schumacher's work fine. The oldest being nearly 30 yrs old. Thant said, a numbers of years ago I purchased a Schumacher "Smart Battery" diagnosis/charger which is junk. It tells me that any battery I connect it to is either sulpherized or beyond recharging.

      To bad that I can't help your post by adding pics of my work, inspired by you.

    5. Steven,

      I am not sure how to enable photos on comments but will give it a look. My favorite charger at home is actually a CTEK but it twice the price of the Battery Tender. I also wanted to power the charger from 240VAC and another reason I ended up with the Battery Tender. I liked that the Battery Tender 800 is fully encapsulated and waterproof to 1 meter, so rain is not an issue. It has been working for about a year or so now.

  2. Hi, Great article!! I followed your instructions to the 't' and it works great. The biggest problems were:
    1) Getting to the control panel as those bolts were rusted.
    2) Getting the kids to allow me to turn off power to the house for 30 minutes :)

    If I could make a brief suggestion, perhaps add a note to warn people about that large capacitor on the built-in trickle charger. My charger was still live (although anemic).


    1. Jeff,

      Describing my post "instructions" is a stretch but I am glad you found the information useful. I didn't mess with the built in charger at all. I just connected the BatteryTender output terminals to the battery. I recommend working with only one battery terminal at a time and treating all terminals as live all the time! The replacement charger has been working well for around a year now. I hope you get long service from your charger upgrade.

    2. Jeff Ginsburg,
      Great point. I'm self taught around electricity, but safety is ALWAYS first. For example, yesterday, I replaced my inter connected whole house smoke alarms, adding 2 smoke/CO alarms. I replaced 14 yr old BRK smoke alarms with BRK/First Alert alarms.
      Thankfully, all "plug and play" since BRK has kept same in wall connector. Next step is to add attic and garage units, but I'll have to shut down the electricity for that. The point is SAFETY First. If you are NOT knowledgeable, and comfortable with that, then call an expert.

  3. Shane is there a video out there that you know of that actually shows you step by step? Thank you

    1. Julian,

      I do not know of a video but will share the link if I find one. Or if someone sends me one. Again, I do not recommend attempting to install an external battery charger yourself unless you are very confident in your ability to not kill yourself working with mains voltages. I expect a service tech or generator repair person could do the install in an hour or less. Money well spent in my mind.


  4. Shane, what should be the input voltage for the battery charger that's on the generac (the old one), I have the same issue but am not getting voltage into the battery charger board. There's 2 spots for wires on the board, and I believe the smaller set is for output. What's your thoughts?

    1. Amelia,

      I know very little about the factor battery charger other than it failed multiple times on my father-in-law's generator. That is why I choose to replace the battery charger with an external unit.

  5. I'm not an EE but I am a capable home electrician. It appears I could simply run a circuit to the generator and plug in a Battery Tender instead of trying to use the built-in wiring - correct?

  6. Ken, running a circuit to the generator should work fine.

  7. Shane, Thanks for the insights on your project. As with many Generac owners I too have had my share of battery charging issues, including an exploding battery which has left my machine case with a bad case of corrosion. I am very fond of my battery Minder Plus which has been keeping my motorcycle battery charged flawlessly for many years. I'm contemplating installing one into my generator, but the charging energy from the Generac battery charger is a constant 15V (probably why my battery blew up here in the Florida heat). The voltage comes from the positive battery lead and prior to that from the starter motor solenoid. I can't install the new charger until I eliminate the voltage from the old one. So I guess my question is, does the new and improved charger from Generac directly replace the old one and if so do you know the part number?


    1. Keith, I know very little about the Generac battery chargers and part numbers. But 15V is much too high for a float voltage in Miami. You should be able to look in the owner's manual for your generator to see how to disable the internal charger.

    2. Thanks Shane, I was able to find the link and part number at Ziller Electric's Forum. I was able to find Generac's installation instructions for the new charger here: http://zillerstore.com/forums/attachment.php?attachmentid=239&d=1323713090. The part number is 0G8023. Many available on Ebay, Amazon, and Genset supply houses. Some have said that this is Generac's smoking gun on battery charger issues for pre 2008 models. I spent the big bucks on an Optima battery. Otima recommends 13.4 volts at 1 amp. The new and improved Generac charger puts out 13.5 V at 2.5 amps. Still too much juice for the Florida heat. For that reason I'm going to install my own "Battery Minder Plus" utilizing the instructions from Generac to preserve the charging solenoid function during run time. Otherwise you must run a circuit from a source which is supplied by the generator when it is running. I hope this will alleviate the overheating issue and preserve my battery so I'm ready for the next hurricane.

  8. Shane I have a 15 kW generac generator that is not charging battery. I got it this way. The charging relay is burn out. And 5amp fuse blown what might have caused this? The relay and transformer were replaced thinking that was the issue. Not by me. I was told it started ran fine check voltage and noticed wires from relay started to get hot and smolder.

    1. Michael, smoldering wires are a GREAT reason for a visit from a qualified service technician! It sounds like a serious overload if not a short circuit condition. Once the generator has passed a safety inspection, you can tackle the charging issue. Again, I found enough references to poor charging to convince me to use an external charger. Good luck!

  9. Please can someone answer this question. I have a Generac XP 8500 that puts out like 8500 watts. Got it converted to gas. Stupid Generac model will not charge its own battery. There is a little wall wart charger you use and plug into the generators ac outlet. So I wanted to install a nice charger to the start battery and so when Im running the generator, I would just plug in the portable charger and charge start battery. I tried that today, Charger showed battery charging, but charger indicators never moved. I finally unhooked all that crap and took battery and charger back to house and hooked it up and battery showed fully charged. Whats going on? Is the electricity output from my generators ac 120 outlets that dirty a charger will not work properly?

  10. Jeff,
    It sounds like your battery was actually charging since it showed fully charged when you took it back to the house. Was the battery almost fully charged when you started? I have had issues diagnosing solar powered charging systems because the battery was too full to draw current from the charging circuit. That may be why your charging indicators never moved. Or there may be a mismatch between your chargers status indicators and your battery type.

  11. Hi Shane i have a 12K gaurdian and when the 5amp fuse is in it make a humming noise when i take it out it stops would that indicate the charging pack is shot it is an older unit like those mentioned here any info would be great thanks

  12. Chris,

    Sorry but I do not know enough about these systems to offer an opinion. You might look for more specialized forums or check with a generator service technician.

  13. Shane,

    Thanks for the great write-up, this looks like a perfect economical solution to the charging problem with Generac generators. I have just diagnosed my own Generac 12kW generator with this same issue. My 3 month old battery was stone cold dead. After giving it a full charge, I then noted that there was 67mA being drawn out of the battery when the generator was on standby. The voltage also dropped from 12.93V open-circuit to 12.64V when connected to the generator on standby. I'm going to order the Battery Tender 800 and install it next weekend.

    I was looking at the model which you noted as the "new" version above (022-0203-DL-WH). It's listed as a Lithium battery charger and I noticed these notes in the description on Amazon:

    *Designed for charging and maintaining Any 4-cell Lithium Power Sport Battery.
    *Programmed Specifically for LifePO4 (Lithium Iron Phosphate) Batteries.

    It looks like this is not a direct replacement for 022-0150-DL-WH and should only be used with Lithium Iron Phosphate batteries. What do you think?

    Gary C.

    1. Gary,

      I grabbed the wrong photo of the replacement charger (now corrected). Thank you for the heads up about the image. The linked charger was always correct for charging 12V lead-acid batteries. You should always verify your charger is a match for your input voltage, battery chemistry and battery voltage. I hope your charger project goes well!

  14. Shane - is this still working with your wife's parents 15 kw generac generator????


    1. Yes, the charger is still working! Updated the post to reflect. Thanks!

  15. This aftermarket charger solution will work but keep in mind, during utility loss, there's no charging. While the outage is happening, the board is not powering the voltage regulator, but, it is energizing the fuel solenoid, and the ATS relay (23 and 194) and it's grounding 23 so that might cause the battery to eventually drain during an extended length outage. This would likely take almost a week to drain that battery though, I would think. I am not really sure what the draw is from the board during outages but I don't think it's all that much. Then again, I have heard of someone killing their car battery by leaving the dome light on overnight, just saying...

    The OEM setup has a BCR (battery charger relay) which switches source power for the battery charger (16vac) over from the transformer, to the battery charge winding on the stator, on these older units that utilize this 16vac charger and the transformer set up. There was some reason why they wanted to power up the charger while the unit was running during an outage, so I can only assume it's because the board would drain the battery, until the unit shut down. On the next gen units, they got rid of the transformer and BCR, and utilized a 120vac, 13.5vdc 1.75 amp charger that was either mounted in the ATS or in generator and was powered off load side of the ATS, so it always had power source even during power outages.

    So if you're concerned about that scenario, then I'd say go old school and install a breaker in the load side of your ATS powered load center, run that circuit out to your generator, and that becomes your "always powered" circuit for the new battery tender. But in all likelihood you won't kill your battery for the typical several hour long power outage. But if you're a prepper...

    1. Thanks for the observations. I understand the needed to charge during an outage and I like the idea of powering the charger directly from the load side. I wondered about running an outdoor rated power cord from the closest outlet to the generator box but that seems like a kludge and expect that is against some code somewhere. I like the idea of having an electrician install an outdoor rated outlet (with a rain shield) close enough to the charger that you can plug the charger into this new outlet. The Battery Tender charger mentioned here is waterproof so you only need to ensure the AC input and battery terminals stay dry. This would let you charge the generator from the generator output during outages.

  16. Shane, great info on Generac charging issues. The older units seem to be prone to charging issues. I have a friend that has a older 13k that cranks a long time before starting. New plugs helped, but still cranking a long time. Sometimes several cycles. Any suggestions or places to find info?

    1. Jeremy, I really am more of an electrical guy!. But I expect that a generator service technician could give you some recommendations. Or anyone who regularly works on small engines. An issue with an automatic choke or cold-start valve comes to mind but I do not even know if the Generac has either. Good luck improving the starting and thanks for the comment!

  17. Thank you Shane! This is exactly what I needed. I have a generator that appears identical to the picture in this article. I have been wanting to install a trickle charger because the generator service company has been replacing the battery almost annually. This solves a lot of my issues. But it raises a question perhaps you could answer. Even though I may never need it, me being as I am, I would want the battery to charge in the event of a long term outage. My question is, do you know how to test the OEM charger to tell if it is working or not?

    1. Jeff, to run the charger during an outage I recommend having an election install an outdoor rated 120VAC outlet on the load side of your Automatic Transfer Switch close enough to the generator that you can plug your waterproof charger directly into the new outlet. This will power the charger from the generator output while the incoming AC is out and from incoming AC when utility power is available.

      Testing the charger generally consists of draining the battery a little and providing a light load and watching the terminal voltage to see if is is drifting up or down. Drifting up means it is charging. Drifting down means either the charger is not working or your load exceeds the charging current capacity of the charger. Note the battery voltage can also drop if you have used an external charger that charges the battery to a final voltage above the generator's float charger voltage. A cyclic charger usually has a higher end of charge voltage compared to a float charger. Charging batteries is much more complex that it looks at first glance.

      Testing the charger on my father-in-law's generator was pretty easy since the charger was not working and the battery terminal voltage was already low, under 12 volts. If the charger was working the battery voltage should have been above 13 volts. You could drain the battery (starting the generator a few times?) enough to pull the terminal voltage below 13 volts and then watch the battery voltage. You should give it a few minutes before looking at the voltage trend since battery terminal voltage can rise after removing a load. But it should be pretty clear if the charger is trying to bring the battery voltage back above 13 volts.

      You can disconnect the incoming AC via the ATS and watch your battery voltage while the generator is running to test the OEM charger to see if the charger, relay and charging winding are working properly. If the battery terminal voltage is increasing, the charger is working. And if the voltage is dropping, the charger or relay or charge winding are bad.

      Here is a great page with information on charging lead-acid batteries.


  18. Hey Guys, I have followed this string for a while and am struggling to understand why the Florida guys have not yet suggested a small solar cell to charge the gen battery. I have one on my generator as well as on my small boat that has no dockside power. The unit I use is intentionally small, less then 10 watts to preclude the need for a charge controller. My generator has been on line for two years and my boat has been in service for 5+years with two jet Skye batteries. The secret is to never let the batteries become fully discharged, that is usually the beginning of the end of any lead acid battery. I purchased my solar charger in the boat section at Amazon for about $25. A tip, be sure to set the solar charger at a bit of an angle to the south so water runs off the glass. If you set it flat, rain water will become trapped by the bezel and leak into the cell.