Inverter power issue

[Mike C 901]

Hi, new member here. My work vehicle is a 2020 Tahoe PPV with the aux battery that is connected to the main battery via an isolator relay while running. We have a 3000w inverter our shop installed and I think they wired it directly to the aux battery, looks to be around 1/0 or so. The issue I have is at idle, I'm showing 12.8VDC at the inverter input while the inverter switched off. We are using the inverter to charge a drone battery bank which looks to be around 1080 watts. After switching the inverter on and then the charge bank, I get a low voltage alarm after about a minute or so. I checked it with my Fluke and it drops to 10.4 at the input at idle. Any suggestions as to why I'm getting such a drop? The feed run is pretty long and I don't know the specs or particularly trust the aux battery.

Thanks for any help.

 

[justinm001]

At idle likely your alternator isn't running so it's either your battery can't handle the amp draw or your cable isn't thick enough. If you measure the voltage at the battery terminal and it drops then its the battery

 

[Mike C 901]

At idle likely your alternator isn't running so it's either your battery can't handle the amp draw or your cable isn't thick enough. If you measure the voltage at the battery terminal and it drops then its the battery

Thanks for the info Justin, I've give that a try. Also forgot to mention, I can hold the engine rpm at 2000 and still get the low input alarm. I'll get my co-worker to hold it above idle and take measurements and also check VDC at the battery!

 

[Tomthumb62]

Also, I'm assuming both your starter battery and aux battery are lead acid. Lead acid is nortorious for voltage drop. You could triple your cable size and if the battery is the issue, it won't help much to your voltage drop issue.

How old is the aux battery?
What is it's charging source other than the alternator?

If no other charging source than the alternator, then there's a high likely chance that the aux battery is severely damaged/sulfated. It either needs a good charge from a real battery charger and/or a desulfator (only if flooded lead acid, not good for AGM/GEL) or needs to be replaced and maintained with a proper charger.

If you can find the specs on the aux battery that would be great. Or the brand and model # and we can look it up online. If it's not a true deep cycle battery, then it might not be up to the task. Starter lead acid batteries are great for short bursts (a few seconds) of hundreds of amps, just enough to start an engine, but very poor at longer periods of draw. Your inverter will be drawing 1080W / 12.8V / 0.85 inverter efficiency = 99.2A. You need a true deep cycle battery such as RV stores will sell. Note that "Marine Deep-Cycle" or "Marine Dual-Cycle" are NOT true deep cycle lead acid batteries. They are essentially a starting battery with a tiny bit of reserve...better than a regular starter battery but not by much.

Lead acid batteries and especially AGM need special charging conditions. They need a bulk charge time, then a long absorb time (usually about 2 hours for a 100Ah 12v AGM) before they go into the float period to finish off the last bit of charging. This isn't happening in many vehicle situations, because the engine isn't running long enough, at high enough RPM's, consistently, for a real charging cycle to occur. Driving for 4+ hours at constant highway speed would be one way to do it, but if that's not the vehicle's daily use pattern, lead acid batteries can die an early death (or simply underperform) unless a quality and appropriately-sized charger is placed on the battery on a regular basis. Which could mean as often as daily (depending on how hard the battery is used) or as little as every 7-14 days.

 

[Mike C 901]

Well, it's a government use vehicle that gets used anywhere between 2 and 16 hours a day given whatever is happening on that day.

It's a factory GM battery, the vehicle is a 2020 with 34k miles that I have been assigned for a full year yet so I don't know the history of the battery. The battery is a group 48PG, 730 CCA and the tag says 20 hr capacity is 70Ah and load test 365. The main battery in the vehicle is unknown due to GM loving to stick electronics packs on top of them. Replacement part shows to be a 94R rated at 80Ah.

 

[Tomthumb62]

The battery is a group 48PG, 730 CCA and the tag says 20 hr capacity is 70Ah and load test 365.

Going with the 70Ah number, at 1080W / 12.8V / 0.85 you again get 99.2A. Which for a healthy lead acid battery shouldn't introduce too much voltage drop, but if it's worn/used/abused due to poor charging conditions, you could get quite a lot of voltage drop. When voltage drop happens, the amps required will go up...which can cause even more voltage drop (and more rise in required amp)...it's vicious cycle. A repaired or new battery would be the fix in this situation. And with such a large inverter, I would double or triple the aux battery capacity, so 200-300Ah would be good.

There are two MAJOR advantages of lithium batteries over lead acid:

1) far less voltage drop. Lithium can take much more amp draw and the voltage will drop only a little and no vicious cycle like lead acid.
2) far less finicky on how they are charged. Lead acid wants to be charged to 100%, and stay there, all the time. They dislike being discharged and then not fully charged back up (hard to get a full charge on alternator alone). Lithium will take 1A of charge or whatever the max charging rate (C rate), which for a 70Ah lithium would be usually a max of 35A (0.5C rate), but it will be totally happy with a rate less than that. And lithium prefers to not be charged to 100% all the time, so the short/incomplete charge cycles from an alternator (and one that varies in output due to up and down of the RPM's of the engine) are just a-ok with lithium. Only catch is lithium cannot be easily or safely charged directly from the alternator (could burn the alternator out) so you need an appropriate DC-DC charger, like this one.

Probably not the sort of thing a gov't agency, who are famous for being stuck in the past, is going to pony up for, though. Even though it could save them money in the long run (2000-4000 cycles of lithium vs 200-400 cycles of lead acid and could be 100 cycles or less if battery is abused/improperly charged regularly).

 

[Mike C 901]

Probably not the sort of thing a gov't agency, who are famous for being stuck in the past, is going to pony up for, though. Even though it could save them money in the long run (2000-4000 cycles of lithium vs 200-400 cycles of lead acid and could be 100 cycles or less if battery is abused/improperly charged regularly).

yeah, these folks don't even like the Cloud... I checked the voltage at the battery and then at the inverter input and I'm losing .7 - .9 between the battery and the inverter and running at idle with no accessory draw, It's still on 12.8. I thought constant output from an alternator was 13.8 in most applications?

 

[Tomthumb62]

yeah, these folks don't even like the Cloud... I checked the voltage at the battery and then at the inverter input and I'm losing .7 - .9 between the battery and the inverter and running at idle with no accessory draw, It's still on 12.8. I thought constant output from an alternator was 13.8 in most applications?

I’m not an expert on alternators, but I think that if you’re just idling the engine, it may not be able to produce enough amps for the larger power draws.

 

[mikefitz]

The auxiliary battery is not being charged by the alternator, faulty alternator, faulty isolating relay , faulty connections, faulty fuse. A check with a meter should find the fault. Need to check alternator output volts and main starter battery volts.
Typical alternator voltage, conventional charging 13.8 to 14.4 volts, 'smart' energy saving charging 12.5 to 13.5 with boosts to near 15 volts

 

[Mike C 901]

I’m not an expert on alternators, but I think that if you’re just idling the engine, it may not be able to produce enough amps for the larger power draws.

i used a prop rod to hold the rpm around 1800 and no change on vdc

 

[Mike C 901]

The auxiliary battery is not being charged by the alternator, faulty alternator, faulty isolating relay , faulty connections, faulty fuse. A check with a meter should find the fault. Need to check alternator output volts and main starter battery volts.
Typical alternator voltage, conventional charging 13.8 to 14.4 volts, 'smart' energy saving charging 12.5 to 13.5 with boosts to near 15 volts

all my other accessories (aux lights, surface pro charger, siren, etc) still work fine, inverter has been installed for about 10 months now. I'm going to see if they will run it to the main battery. I never use it without the vehicle running so killing start power shouldn't be an issue.

 

[Mike C 901]

just an update, i ran the truck to the shop and they replaced the aux battery and nothing changed. We checked everything again and are getting a huge loss over the length of the run. I also found out they only used 4GA when it calls for a minimum of 2. They are rewiring it next week and we shall see how it goes. Next step will be a rear battery setup they say.

 

[Mike C 901]

Got the truck re-wired today, should be the equivalent of 1/0. Appears to have fixed the issue as I ran 3 chargers with it at the same time and had no issues.

 

[Tomthumb62]

should be the equivalent of 1/0.

Yeah 12v stuff is no joke, lol. Thanks for the update and glad things are working now.