Is there a rule of thumb or guidance for an expected voltage drop based on amp load on a pair of 6V GC batteries? I was tinkering in my trailer this weekend and ran a 750 watt AC electric heater off of my inverter. Batteries seemed fully charged prior to this based on voltage readings, but no SG reading taken. I had a much larger voltage drop than expected, it was pulled down to 10.5V and the inverter was giving an alarm. I assume due to low voltage. Although the whole time the heater was operating and I was getting hot air out of it. I let the heater run for only about 15-30 seconds. Ambient temp was around freezing (32F). I only did this experiment out of curiosity and not out of any need. Sometimes I'm just bored and want to tinker in the camper! Inverter wired with 0 gauge wiring, about 5' from batteries. Voltage came back up to about 12.5V after removing all loads.I think I've done this same thing before and did not have such a large voltage drop. Due to the trailer being in the repair shop all summer and the batteries being in various states of disconnect/connected but no solar/solar on but parked inside, I think my batteries took a bit of a hammering. They were like 10.5V when I hooked up the trailer to bring home. I know that's totally dead for a 12V system, but they are deep cycle and this is the first time they were ever down that low. Batteries are 2 years old. I was going to wait until spring to replace them if needed, but I also thought they seemed to be behaving fine after several days of shore-power charging and then letting my solar take over. But it does seem like they are showing a larger voltage drop for small draws (like LED lights) than I remember before this summer.

6v GC have higher signficantly internal resistance and hence voltage drop than do 12V starting or marine or for that matter deep cycle batteries. 750W load is around 60-70A, a pretty good load for a pair of GC.Here is what I and some others have found with GC batteries. Once they get below about 80%SOC, a load in the 750-1000W range WILL very often trigger the low voltage alarm on a inverter and have a voltage in the 10.5ish range. The fact that it rose to 12.5 volts w/o a load is IMHO a good indicator that the batteries are probably around the 75-80% SOC and you just found the reason that those who draw 75+ Amps from a battery bank either go with mutiple 12V or a bank of 4 GC. It also doesn't help that you were around freezing. AH capacity of a battery decreases at low temps, and internal resistance rises. All going in the WRONG direction if your trying to get a big draw from GC batteries. So.... before you get real worried, get the batteries FULLY charged with a good charger, measure the SG on each cell. Good chance what you will find is that you know the main drawback to GC batteries.4 GC with 550AH capacity won't give the cranking amps that I get out of a puny 70AH 12V starting battery for my diesel.

Seems like the inverter and wiring are ok but the batteries are at reduced capacity from the described situation they have been in.First they need a real good charge to as near full as possible, then a good high voltage desulfation session to get them back as close as possible to proper SG.Then see if they will "hold a charge" well enough for what you need them for, including with inverter loads.

Thanks for the responses. I did suspect that the cold temps and higher resistance of 6V might be culprits for what I observed. The last time I did this experiment I do think I was still running 2x12V marine batteries. So that could explain me not remembering any low voltage issues. I didn't take any SG readings prior to experiment due to darkness and laziness, nor afterwards. 12"+ new snow over night, I'm not going to unbury anything anytime soon to take readings!

A pair of good 6s should do better than what was reported. It is true that 12s and AGMs do better for voltage sag under load than 6s do, but that is not the real issue here IMO.Those poor batteries have been neglected for several months, and now they are going to be neglected some more, due to the situation. Perhaps it can't be helped, but IMO expect to be buying new batteries in the spring if you can't get them in and do some TLC on them (which might not work by now, but is worth a try for sure)We hear about his all the time it seems. RVs going in the shop and coming out later with their batteries shot. It adds to the repair bill. I think I would try to take my batteries home and get them to use one of the shop's batteries while they have the rig, based on all the stories like this.

expected normal voltage drop from load? | Good Sam Community

29 Replies

ktmrfs
Explorer II
Dec 14, 2015
5' of #0 shouldn't give excessive drop for this current. Should be about 0.001 ohms (1 milliohm) resistance, x 75A is 0.075V drop little less than a tenth of a volt.

I still suspect that the biggest issue is the cold temps and something near 80% or less SOC. That combo with a pair of GC is marginal. I'd try a good charge and equalization cycle followed by measuring SG to see where you are.

Again, GC are not a good choice for continous high current draws. That's a job for good old 12V jars with very low internal resistance designed to handle very high currents.
ewarnerusa
Nomad
Dec 14, 2015
RoyB,
It is about a 5' wire run with 0 gauge cables. I don't really ever draw that many amps during normal use, I was just out there tinkering because I haven't camped since summer '14 and I miss playing with this thing! I was exercising the inverter, let's say.
ewarnerusa
Nomad
Dec 14, 2015
Agreed BFL13. The annoying thing is that I thought I had done a good job at preventing battery abuse by checking in on them periodically. And they still got abused, my first worry was when I checked on things to winterize in early October and noticed lots of corrosion on the battery terminals that had never been there before. Solar charging voltage looked fine, water levels down a bit but hardly low, but when I plugged my air compressor into the inverter the voltage dropped big time and it faulted out. I had to hook the inverter up to my truck battery to run the air compressor. That's when I first suspected I may have toasted batteries.
RoyB
Explorer II
Dec 14, 2015
Ed - Hope your battery cables are large enough to draw the high DC currents... I am getting away with just running a good solid 20-22 AMPS DC CURRENT from around 5 PM to 11-PM each evening on my off-road trips and my 12VDC batteries (255Ahs total) still holding up pretty good here...

Been doing this since around 2009 and I am just now starting to see my batteries performance just now starting to fall back on me.

I know next season is going to bad for my batteries for me as I didn't get to go out much the past two previous years like I usually do... I will need to replace them for sure...

Roy ken
BFL13
Explorer II
Dec 14, 2015
A pair of good 6s should do better than what was reported. It is true that 12s and AGMs do better for voltage sag under load than 6s do, but that is not the real issue here IMO.

Those poor batteries have been neglected for several months, and now they are going to be neglected some more, due to the situation. Perhaps it can't be helped, but IMO expect to be buying new batteries in the spring if you can't get them in and do some TLC on them (which might not work by now, but is worth a try for sure)

We hear about his all the time it seems. RVs going in the shop and coming out later with their batteries shot. It adds to the repair bill. I think I would try to take my batteries home and get them to use one of the shop's batteries while they have the rig, based on all the stories like this.
ewarnerusa
Nomad
Dec 14, 2015
Thanks for the responses. I did suspect that the cold temps and higher resistance of 6V might be culprits for what I observed. The last time I did this experiment I do think I was still running 2x12V marine batteries. So that could explain me not remembering any low voltage issues.

I didn't take any SG readings prior to experiment due to darkness and laziness, nor afterwards. 12"+ new snow over night, I'm not going to unbury anything anytime soon to take readings!
BFL13
Explorer II
Dec 14, 2015
Seems like the inverter and wiring are ok but the batteries are at reduced capacity from the described situation they have been in.

First they need a real good charge to as near full as possible, then a good high voltage desulfation session to get them back as close as possible to proper SG.

Then see if they will "hold a charge" well enough for what you need them for, including with inverter loads.
ktmrfs
Explorer II
Dec 14, 2015
6v GC have higher signficantly internal resistance and hence voltage drop than do 12V starting or marine or for that matter deep cycle batteries. 750W load is around 60-70A, a pretty good load for a pair of GC.

Here is what I and some others have found with GC batteries. Once they get below about 80%SOC, a load in the 750-1000W range WILL very often trigger the low voltage alarm on a inverter and have a voltage in the 10.5ish range. The fact that it rose to 12.5 volts w/o a load is IMHO a good indicator that the batteries are probably around the 75-80% SOC and you just found the reason that those who draw 75+ Amps from a battery bank either go with mutiple 12V or a bank of 4 GC. It also doesn't help that you were around freezing. AH capacity of a battery decreases at low temps, and internal resistance rises. All going in the WRONG direction if your trying to get a big draw from GC batteries.

So.... before you get real worried, get the batteries FULLY charged with a good charger, measure the SG on each cell. Good chance what you will find is that you know the main drawback to GC batteries.

4 GC with 550AH capacity won't give the cranking amps that I get out of a puny 70AH 12V starting battery for my diesel.
azrving
Explorer
Dec 14, 2015
Is the sg up?

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expected normal voltage drop from load?

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