Keeping Generator Battery Charged

Michael57 · March 2021

Hi All
I've searched around a bit and found some similar questions, but nothing that was quite as specific to my situation, so I'll ask for some advice.

On my system at an off grid weekend cabin, I have a Generac Generator that basically uses a car battery to start its engine. The generator is connected to an AGS will trigger it to run if my solar batteries get low or to exercise it. It generally works really well, but the one challenge is that the Generac generator has a control panel that puts a constant draw on the generator's battery. So if I go too long without the generator being triggered, that battery can eventually be too depleted to start the engine. Today, I deal with this manually by having a battery maintainer and another battery in my garage and just rotate them once a day, but I'd love to have something that is a bit more set and forget.

In my situation, the generator sits just a few feet behind my power shed and my power shed has a 120v outlet. So I was thinking I could just put a battery maintainer in the shed, plugged into the 120v outlet and basically run the charging cables through the back wall and leave them permanently attached to the generator battery. I've also found some marine battery chargers that are meant to be mounted in the engine compartment so they have ignition protection, are waterproof, and are able to tolerate outside weather conditions. I think I could probably fit one of those inside the generator's housing which might be a bit cleaner.

Is there any concern over leaving a devices like these connected to the generator battery full time, even if the generator is running? Are there other better options out there, or anything I'm missing?

Thanks!

BB. · March 2021

Nope--A "good quality" float charger will hold the battery around 13.6 to 13.8 volts +/- a few 1/10's of a volt. And the float charger will not be bothered when the genset's charging circuit brings to ~14.2 to 14.4 volts or so...

When I say "good quality"--Don't get the $15 1 amp float charger cubes... They do not regulate the output current very well and can boil your starting battery "dry" (expose plates). I tried a few "cheap float chargers" on a couple of cars that are not driven much--And "cooked" a few batteries.

Some chargers I have used and work well:

https://www.amazon.com/s?k=battery+minder&ref=nb_sb_noss_2 (good if you need to both "charge" and "float" battery)
https://www.amazon.com/s?k=batter+tender&ref=nb_sb_noss_2 (I have not used--But well respected brand)

https://www.costco.com/duracell-4-amp-battery-charger-and-maintainer.product.100479359.html (Costco online)
https://www.homedepot.com/p/Duracell-4-Amp-Battery-Maintainer-Charger-DRMC4A/314031686 (Home Depot--more $)

Ideally, the charge controller should be the same temperature as the battery bank (i.e. charger not in conditioned space while genset is in 0F to 100F conditions). The charging/floating voltage is normally reduced for flooded cell lead acid batteries as the battery temperature rises above ~75F. Some Battery Minders have an external temperature sensor that you can extend out to the battery compartment (optional extension cable?).

The several "good quality" float chargers have all maintained around 13.6 volts and I have never had to add water to the battery on float (car batteries). The Battery Minder and Duracell do have a 2-4 amp or so charger too--So if you need to "recharge" the Genset Battery, they can do that too...

A few off grid folks have also installed a smallish solar array and simple solar battery charger to keep the genset battery "fresh" with good results too (typically off grid folks that do not have 120 VAC or don't run the AC inverter all the time). A nice solution if you are off grid and don't run the AC inverter when nobody is there.

AC inverters do use a fair amount of amps when running, even if no loads--If you do not need AC loads, it is better to shut down the AC inverter--That way if something "goes wrong"--Genset does not start, Bad Weather, Snow on panels--You don't have loads discharging and killing your battery bank.

-Bill

PS: Be careful with the 12 volt wiring--You don't want sharp sheet metal (genset panels) to cut the 12 volt to charger wire... Adding a fuse/breaker to the +12 wire (at/near the battery terminals) can reduce the chances of fire.

Michael57 · March 2021

Perfect, I actually use the 2A Battery Minder in the garage already for the rotated battery and for my ATV's when they sit for a bit. It also has an optional temperature attachment that I can pick up. For the most part, the power shed will be close to the same temp as the generator, but the big exception to that is when the generator runs as it noticeably heats up the generator compartment, so the temp attachment will be worth it.

When I leave the cabin, I turn off the inverter and disconnect the generator battery. My solar array is on the smaller side for my setup (though sufficient for weekend use), and in western WA during the winter we can get a few days with nearly zero solar generation with the cloud cover and fog that likes to settle in on the cabin, so I just don't trust leaving it on.

I have thought about going the solar route for this, but the back side of my power shed where the generator sits has a bit of tree cover so I'd need a long run which would be a pain when I have a 120v outlet right there. Also, the most common time my generator kicks on is in the morning before the sun is putting out much useful light, and folks are waking up, making coffee, taking showers etc. and I'm not certain a solar float charger would always have that battery ready at that point in the day, especially in winter.

Thanks for the note about the wiring, I was looking at the conduit that's already running between the shed and the generator, but if there is not enough room in that to add another cable I will take care around that sheet metal and see about a fuse near the battery either way.

Thanks for the help Bill!

BB. · March 2021

You are very welcome Michael. Some other comments:

Bolt up connections are more reliable than battery clips...

And following code, 12 VDC and 120 VAC circuits are not supposed to be in the dave conduit (low voltage vs high voltage). If you share different wire voltages in the same run, all insulation should be rated at the higher voltage...

Anyway that be code.

Bill

stillchillin · March 2021

Hi what is the model of your generator? Generac generators typically have a built in battery charger but you have to power up L1 & L2 for it to work.
Steve

Michael57 · March 2021

Thanks for the added comments Bill, I have taken note of that advice.

stillchillin
The Model is 13GSBA-6711B, please let me know if there is a built in charger that I can utilize.

EDIT: I am probably showing my ignorance here, but looking at the manual for the generator, it says that there is a battery charger in the transfer switch for keeping the battery charged, but since we are off grid I don't think there is a transfer switch wired in. Does that make sense?

stillchillin · March 2021

the model # you referenced is a Cummins 13KW generator and you are right about the battery charger being in the transfer switch.
Your idea about installing a trickle charger seems like a simple solution to the dead battery problem. good luck

jtdiesel65 · March 2021

IMO, something is wrong if you have to swap batteries once a day. Something is wrong if you swap in a charged battery as soon as you get there and the battery has no power to start the generator while you are there. Is there a short? Mice? Does the alternator work? I would get a clamp on meter and see what exactly is the draw with the unit off in its "auto" mode. This seems excessive. Kolhers can go weeks without this being a problem.

Simple solution for when you are there is a trickle charger. I guess I would be looking for the solution that allows for not having to disconnect the battery.

I would get a clamp on meter and see what the inverter and trickle charger pull and do some math to see how much it would draw down the battery over time. You may want to leave the inverter on or get a small inverter to just power the charger circuit.

I'd also look into the solar panel route. Even with some shade, it may work fine for it's use. Is one side of your shed facing the sun? You could mount the panel on the shed.

solar_dave · March 2021

BB. said:

Nope--A "good quality" float charger will hold the battery around 13.6 to 13.8 volts +/- a few 1/10's of a volt. And the float charger will not be bothered when the genset's charging circuit brings to ~14.2 to 14.4 volts or so...

When I say "good quality"--Don't get the $15 1 amp float charger cubes... They do not regulate the output current very well and can boil your starting battery "dry" (expose plates). I tried a few "cheap float chargers" on a couple of cars that are not driven much--And "cooked" a few batteries.

<snip>

I had the same problem with inexpensive trickle chargers cooking off the battery. Easy solution was to put a timer to only run the charger a few hours per day. When they first come on they bump the battery up and that should be enough to get a to the next day. I tried several charger NOCO, battery tender ... The battery tender with a timer seems to be the best.

BB. · March 2021

I did the timer too. Usually 1 hour per day worked fine...

Until my in-laws left the trunk cracked and the trunk light on... the timer did not keep up with the draw.

Another time, I had a dead battery (my fault) and I plugged in a cheap 1 amp charger because it was handy. 24 hours later it overheated and split the case. Still worked, but not really a good product either.

Bill

Michael57 · March 2021

j jtdiesel65 said:

IMO, something is wrong if you have to swap batteries once a day. Something is wrong if you swap in a charged battery as soon as you get there and the battery has no power to start the generator while you are there. Is there a short? Mice? Does the alternator work? I would get a clamp on meter and see what exactly is the draw with the unit off in its "auto" mode. This seems excessive. Kolhers can go weeks without this being a problem.

Simple solution for when you are there is a trickle charger. I guess I would be looking for the solution that allows for not having to disconnect the battery.

I would get a clamp on meter and see what the inverter and trickle charger pull and do some math to see how much it would draw down the battery over time. You may want to leave the inverter on or get a small inverter to just power the charger circuit.

I'd also look into the solar panel route. Even with some shade, it may work fine for it's use. Is one side of your shed facing the sun? You could mount the panel on the shed.

The point of the trickle charger solution I mentioned would be so that I don't have to disconnect the generator battery while I am there. The battery would basically be always connected to the charger and charging as long as the inverter is on. I'd run the charging cable from the power shed right into the generator's case, so I never have to touch it unless I'm getting into the generator for some reason. What would be an advantage of using a solar charger versus an AC charger? To me it seems like it would be less reliable, but I certainly could be missing something.

Going from memory, I think the generator's LCD panel shows the battery going from ~12.6v to ~11.9v after about 15 hours. I don't remember the AH spec of the battery though. I don't think there's an issue with the alternator as it does seem to fully charge the battery when the generator runs. I'll see if I can measure the draw the next time I am there, I have never checked that.

BB. · March 2021

It could be a weak/failing or the electronics for the auto start and transfer functions just draws a fair amount of current. Not designed for off grid usage... Assumes AC line voltage unless there is a mains failure then starts genset.

Measure the idle system current?

Bill

jtdiesel65 · March 2021

I was thinking that the solar charger would allow you not have to disconnect the battery when you leave. But it sounds like the electronics use a ton of juice.

Michael57 · March 2021

Ah yes, that makes sense. That would be an improvement, but it is pretty easy to disconnect, so not a priority to solve right now I am curious about how much the electronics are using, so I'll try to put a clamp meter on it while idle, the next time I am there.

BB. · March 2021

Remember AC+DC Current Clamp DMM... Not an AC only CC meter.

Also, what is the AH rating of the battery? And type of battery?

-Bill

Michael57 · March 2021

Yup, my clamp meter does both AC and DC (Klein CL800, based on your recommendation

), thanks for the reminder. I don't remember off the top of my head what the AH rating on the batteries is (I want to say one of them is 80...but I could be wrong). The batteries are different, but the one I typically use is a Napa Legend car battery the other is a Cummins branded battery that they left with me when they serviced the generator right after I purchased the property. I may be at the property this weekend and I'll check all of this for both batteries and validate how fast the battery is losing voltage.

The last time I was at the property, I did hook up the battery minder to the battery while it was connected to the generator, just by running an extension cord outside the power shed, leaving the generator lid open, and using the battery clamps, so not a long term solution. I wasn't sure if it would be safe to leave it connected if the generator ran and was also charging the battery, so I removed it before that could happen.

Anyhow, according to the generator's LCD panel it did slowly charge the battery. Given that it is a 2A charger, then I'm guessing the draw of the LCD panel must be less than 2A, right?. Should that be enough to draw the battery down to the point that it can't start the generator in 24hours? Maybe the battery is just not in good shape?

jtdiesel65 · March 2021

The battery tenders and solar chargers I've used for motorcycles and atvs, I have just left on while running with no problem. But that was just short term when working on the bikes. My current kohler automatically turns off the battery charger before the starter circuit is engaged and then turns it back on when the unit stops. It might depend on the charger.

If this is the right unit,

https://powersuite.cummins.com/PS5/PS5Content/SiteContent/en/Binary_Asset/pdf/Commercial/SparkIgnited/a-1559.pdf ,

then

Battery requirement

• 12 V, group 26R, 545 CCA Maintenance Free

and

Transfer switch (also sold separately)

• 4 amp trickle battery charger included

I bet they undersize the battery expecting the charger to carry the load.

80 ah battery should run 2 amp load for 40 hrs right? 2amps * 40 hrs = 80 amp-hrs

2 amps for 24 hrs is 48 ah. That doesn't seem to jive with the voltage measurements. I think you should measure things to see where everything stands and consider a 4 amp trickle charger instead of 2 since that is what Cummins uses.

Michael57 · March 2021

Agreed, I am going to measure and validate what I am seeing. Thanks for checking on it and recommending the 4A charger, that seems like a good idea.

BB. · March 2021

Running parallel battery chargers (AC, Solar, motor alternator, etc.) should not be a problem (assuming all are working correctly and programmed for the right Absorb/Float voltages). The charger with the highest "setpoint" wins.

60-80 AH for a "typical" car battery is probably correct. HOWEVER:

SLI (starting lighting ignition) batteries are not designed to deep cycle. They are generally expected to run from 85% to 100% State of charge--Cycling below 85% can cause them an early death.

Lead Acid Deep Cycle batteries can be cycled deeply (50% or even to 20%)--And they are OK as long as they are quickly/fully recharged.

And deep cycle lead acid batteries are generally cycled from 100% to 75% state of charge (storage, small discharges over time)... At ~75% state of charge, then the battery is recharged back to full... If left to 75% or less SoC (days/weeks/months), the battery will relatively quickly sulfate and permanently lose capacity/die.

Say your auto transfer switch takes 1 amp for the idle state (monitoring line voltage, possibly autostart logic, etc.):

SLI Battery: 80 AH * 15% max discharge = 12 AH of "suggested standby capacity"
Deep Cycle LA: 80 AH * 25% max discharge = 20 AH of "suggested standby capacity"
Battery to dead: 80 AH / 1 Amp idle current = 80 Hours of "standby to dead"

IF (a "big if"), there is significant standby current on the auto start/transfer switch--Then you may need a larger battery bank and charger (for example solar charging) or a more permanent/stable charging power source (your house batteries with a DC to DC charger, or similar) to keep the genst battery "fresh".

As you can see, even a 1 amp draw is not even suggested for a single car size battery for a 1/2 day or a bit more between "charging sessions".

Regarding the question of turning of a charger before starting (like the Kohler)--It depends on the charger itself. A good quality charger will limits output current to its rated output current. Even if the battery is drawn to 11 or 10 volts during cranking.

A "cheap" charger may not limit its output current and be damaged over time if it tries to surge its output during cranking (such as my charging and overheating a dead battery on my van with a $15 1 amp charger cube because it was handy).

Note since they are using a "maintenance free" battery (same as typical car battery?)--You want to keep the float voltage relatively low (i.e. 13.6 volts or so)... If you have a "cube charger" that wants to keep 14.0 to 14.4+ volts on "float", it will probably gas the battery and expose the plates (bad) or need refilled with distilled water--If you can pop the capts to refill (my experience with the "cheap" float chargers--Kept "boiling" the batteries and exposing the plates in a few months until a bought more expensive float chargers).

-Bill

Michael57 · March 2021

I was able to get a clamp meter on the line from the generator starter battery a few times this weekend while it was idle, each time the meter showed about .5A of current. So I don't think there is another issue affecting the draw on the starter battery.

BB. · March 2021

A 0.5 amp "vampire" draw is pretty significant...

80 AH * 0.15 "SLI design cycle depth" = 12 AH between charges for best battery life
12 AH / 0.5 amps = 24 Hours between charging... Not great
0.5 Amps * 24 hours per day = 12 Watt*Hours per day

If you wanted to use solar charging... Don't remember/know where your cabin is located--Say has 2 hour minimum sun per day in winter:

12 WH per day * 1/0.61 DC solar system eff * 1/2 hours of sun per day = 10 Watt panel minimum

Charging an 80 AH @ 12 volt battery:

80 AH * 14.5 volts charging * 1/0.77 panel+controller eff * 0.05 rate of charge = 75 Watt panel minimum
80 AH * 14.5 volts charging * 1/0.77 panel+controller eff * 0.10 rate of charge = 150 Watt panel nominal

The large AH battery in standby... 75 Watt array seems like overkill... The minimum 10 Watt panel * 2 = 20 Watt suggested panel would probably be "good enough" until the battery starts to go bad...

Otherwise a 2 amp @ 12 volt minimum "quality float charger" (120 VAC or XX DC) should work fine.

-Bill

Keeping Generator Battery Charged

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