AGM battery voltage

[Kan2]

Hello,

I'm having a difficult time finding reliable information about what voltage is too low for AGM SLA batteries, I want them to live as long as possible. What voltage should be the absolute minimum?

My system is one poli 12v 100w panel. (Will be 400w and eventually 1200w)
Two 24ah 12v batteries (will eventually be 8+)
30a pwm controller will replace with larger 40a mppt.
2000w/4000w inverter (I don't use anywhere close to the 2000w just yet, mainly to charge my phone and what not)

Any information would be very helpful,

Regards

 

[Rider]

AGMs you don't want to drop below 50% typically, although an occasional drop below that won't significantly hurt. Just try not to do it often or you'll shorten your battery life.
Also, there are a few posts about not exceeding 80-90% of it's max during charging. consistently charging to 100% (over 13VDC) also shortens life.

 

[Kan2]

Another question would be: the 50% is it when under load or idle?

 

[Rider]

Idle. At rest. However, if you are always under load pulling amps you'll never see the 'at rest' value.

 

[Kan2]

Is there a way to know what volt is minimum when under load? Or just use the same chart to play it safe at 50% and no less?

 

[Rider]

I don't have a way, maybe someone else does. But a 50% battery under load can show under 11 VDC. If you play it safe and never let the voltage drop below 50% loaded you're REALLY playing it safe, hardly using the available capacity. if that's ok with your application, great. Your battery will appreciate it.

 

[Kan2]

I understand, this is definitely a hobby right now I'm doing solar because I can. While at the same time trying to use the power on something useful (don't yet know).

It was something worrying me, I saw the vdc go down pretty quickly although I just doubled the bank size, but now I know why.

Thanks so much.

 

[Kan2]

After tonight test: under load I got my bank down to 12.1v, after letting it sit over night it was at 12.5v does that mean that I hardly even used the available capacity? This is confusing me.

Thanks,

 

[Rider]

After tonight test: under load I got my bank down to 12.1v, after letting it sit over night it was at 12.5v does that mean that I hardly even used the available capacity? This is confusing me.

Thanks,

I wouldn't say "hardly used", but usage like that will prolong the battery life. You have some extra power available, and even more if you add more battery like you said.

 

[offgriddle]

The load vs no load measurement is interesting. A battery can read a healthy 12 volts under no load but really be quite discharged and as soon as a modest load is applied, the voltage can drop significantly which definitely means it's time to charge. If a lead acid battery is reading less than 11 volts no load, of course it needs a charge but it's also possible that one or more of the individual cells, (each cell in a lead acid is about 2.2 volts), is weak or dead.