Charging deep cell batteries - a question

ScottBarMtn

Hi,

I've got a question here to make sure I understand what it entails to charge deep cell batteries. Suppose that I have the following:

- Two Trojan T105-RE 6 Volt, 225 AH Deep Cycle Batteries connected in series
- A Xantrex Truecharge2 12-Volt, 20-Amp Battery Charger
- Honda eu2000i generator

Suppose the battery bank above is 50% discharged, there would be a deficit of 112.5 AH that needs recharging.

Given the 20-amp 12-volt battery charger, does this mean that I would have to run the generator for approximately 5.5 hours to fully charge the battery? 20 amps x 5.5 hours yields 110 AH? I'm just not sure if the math is that simple, or if I am missing something...

Thanks.

Denver

BB.

Re: Charging deep cell batteries - a question

Yes, the math is that simple... The practice is not.

First, you will have to return around 110-120% of the AH used (flooded cell batteries are around 80% energy efficient, so you have to return more power than you used).

Second, the batteries will taper down charging current once they are at 80-90% charge--A full charge would take another 2-4 hours (from the genset) to get to ~100% full.

Normally, a genset is used in the morning (before sunup) to recharge to 80-90%, then let the solar array take the last part to 100% full.

Also, you really don't have to get a battery 100% charged every day... If you get it >~90% charged on a regular basis, that is fine.

Full Charge/Equalize is only needed every month or three. Equalization is actually "hard" on a battery bank and should only be done if the battery cells have unbalanced specific gravity (more than 0.030 difference between high an low cells--and should only be done ~ 1 hour at a time, measure with hydrometer, and another hour, check--then stop when the low cells stop increasing S.G.).

The other big reason to use a genset is to get the battery quickly above ~75% state of charge--If the battery spends days/weeks/months below ~75% of charge, it will tend to sulfate and lose capacity (and fail).

You can read these Battery FAQ's for more information:

Deep Cycle Battery FAQ
www.batteryfaq.org

Again, lots of rules of thumb to get you close to understanding the basics of good battery operation. Not carved in stone.

-Bill