# Charging voltage at low temperatures

Solar Expert Posts: 203 ✭✭✭✭✭
We got a nice cold spell recently, down into the 20s F, high in the low 40s. I have my two Trojan T-105 batteries in an uninsulated cargo trailer.

I understand that battery capacity goes down as temperature goes down. Trojan says: "A good rule of thumb is that for every 15º F (9º C) above 80º F (27º C), capacity is increased by 10% and for every 15º F (9º C) below 80º F (27º C), capacity is reduced by 10%."

So my 225 Ah batteries at 40F = 165 Ah.

What I'm less clear on is charging voltage. At 80º F, Trojan says absorption voltage should be 14.8. But voltage should be higher at low temps:
Every 10 degrees below 80º F (27º C), add .028 volt per cell to the charger voltage setting.

Example: A 12 volt battery @ 70º F (21º C). The recommended charging voltage (@ 80º F (27º C)) is 14.80 volts. The adjusted charging voltage is 14.60 +(6 cells * .028vpc) = 14.77 volts.

http://www.trojanbattery.com/tech-support/faq/#Temperature

I have a 12V system, 6 cells, so at 40º F my absorption voltage should be 15.27. This seems high to me, am I missing something?

0.028 (per cell adjustment) * 4 (at 40º F) * 6 cells = 0.672 + 14.60 = 15.272

Not sure why they're using 14.60 instead of 14.80 in the temperature compensation equation.

• Banned Posts: 17,615 ✭✭
Re: Charging voltage at low temperatures

Or: why there are Remote Temperature Sensors for battery banks.

They increase the charging Voltage as temperature goes down, decrease it as it goes up.

As the battery charges, it warms up.

So when you hit Absorb internal battery temperature will definitely not be the same as ambient.

It is this internal temperature that matters. Ambient temperature will have some effect on it but they are not equal.

In other words, use an RTS and don't worry about it.
• Solar Expert Posts: 203 ✭✭✭✭✭
Re: Charging voltage at low temperatures

An RTS is on my list, right after a battery meter. But I'm in a "no unnecessary expenditures" mode as the farm season unwinds and my monthly income drops.

In the mean time, running this system manually, should I have absorption voltage set to 15.3ish when ambient temp is 40 F?

Forgot to add that I'm mostly using the generator lately due to very poor insolation (clouds and unexpected shading from trees as the sun progressively gets lower).
• Banned Posts: 17,615 ✭✭
Re: Charging voltage at low temperatures

Charge at normal Voltage setting, check SG. If it's too low increase the Voltage by 0.2 and repeat. You should get a workable Voltage in a few cycles. Just remember you have to turn it back down if it warms up.
• Solar Expert Posts: 5,183 ✭✭✭✭

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• Solar Expert Posts: 203 ✭✭✭✭✭
Re: Charging voltage at low temperatures
westbranch wrote: »
PMan, RTS are valued between \$20 - \$30, a good battery monitor is in the \$200 - \$300 range
Don't think an RTS will do much of anything right now since I'm primarily using the generator + Iota to charge my batts.

I would definitely like a battery meter. But do you have idea how many carrots I have to bunch to make \$200-\$300? I shudder to think. Next year maybe, if I'm still living off grid.
• Solar Expert Posts: 203 ✭✭✭✭✭
Re: Charging voltage at low temperatures
Charge at normal Voltage setting, check SG. If it's too low increase the Voltage by 0.2 and repeat. You should get a workable Voltage in a few cycles. Just remember you have to turn it back down if it warms up.

Well the SG has generally been quite a bit lower lately compared to summer. I think I'll step up the voltage.
• Solar Expert Posts: 175 ✭✭✭
Re: Charging voltage at low temperatures
Plowman wrote: »
Don't think an RTS will do much of anything right now since I'm primarily using the generator + Iota to charge my batts.

I would definitely like a battery meter. But do you have idea how many carrots I have to bunch to make \$200-\$300? I shudder to think. Next year maybe, if I'm still living off grid.
Depending on the model the Iota should have RTS capability.
• Solar Expert Posts: 61 ✭✭✭✭
Re: Charging voltage at low temperatures

Going to piggyback on this if OP doesn't mind. Its a debate but what's the optimal placement for RTS? I've put mine in the middle of the bank attached to the battery's plastic casing mid way up.
Oddly even through extended absorbs I've never felt a temperature difference on the plastic case