# Importance of Absorption voltage and SOC?

How important is the absorption voltage in the charge 'finish' process, compared to the bulk charge voltage?

For example, lets say a pair of t-105's in series should have a charge voltage of 14.8 volts. (or whatever the mfg. recommends) And at about 80% of charge, that voltage is hit, and the charger drops into the absorption stage, and finishes off the charge.

What happens if the absorption stage voltage drops, and is 14.2vdc? .6 volts lower. Given battery chemistry, is that lower voltage going to push that last 2-5% of charge into the battery?

Does it just take a longer time to hit 100%, or is it always finishing at lets say, 95%?

I'm just curious, but that's what my Iota DLS-45/IQ4 does. Bulk charges till it hits 14.76, then drops to absorption at 14.16vdc. It may be settable inside, but I haven't found any documentation on that.

I have not had _any_ issues with my batteries, and they're not Trojans, that was just an example, but it came to my attention because my Sunsaver MPPT does bulk charge up to the absorption voltage level, and stays at that voltage until it's done (tapering off on current). So if I set my absorption to 14.8, it would finish there.

I don't think I'll loose any sleep over it, but I couldn't find an answer either...

Thanks
Greg

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Re: Importance of Absorption voltage and SOC?

You've pretty much got it: longer time at lower Voltage.
But keep in mind that Trojan's recommended 14.8 Volts is on their meter, not yours. You can put two different meters on the same battery at the same time and get two different readings. As a general rule, Absorption Voltage for "12 Volt" systems is 14.2-14.4. Interstate has really high numbers. A few tenths of a Volt doesn't make a big difference in the immediate scheme of things.

Now for the next answer from someone else which will say just the opposite. :roll:
It should all be science, but it's still a little bit art. Or maybe voodoo.
Re: Importance of Absorption voltage and SOC?

When charging with the sun and a fixed solar array, you have a limited amount of time to recharge the battery bank. Usually, you need to push the energy back in as quickly as possible (and run 14.5+ volts to do that).

Some controllers time the absorb charge time (~1-4 hours for most people). Others measure the current and when it is below ~2% of rate of charge--they drop back to float (I think I have that right)...

Each method has its pluses and minuses... And lots of friendly discussions about ideal charging voltages/currents, mixing of electrolyte, how often to equalize, etc.

One user here (Dave Sparks) is a huge proponent of solar trackers because it gives you more hours to recharge your battery bank and you can do a full charge with less current/absorb voltage--His experience is that this gives him a longer battery life. (and his customers are happy too).

Lots of trade offs between time, money, and more money. :roll:;)

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset