Odd battery behavior

[bobbyq]

I'm relatively new to the solar world. I have a Rich MPPT 60 amp charge controller. I have observed that with all loads removed and investor turned off my 24 volt battery bank will drop from 100% to around 85% within 15 minutes of losing direct sun. By 9:00 PM it is down to 65%, however it powers my fridge all night with TV and fan on till I go to bed. It is around 60% when I get up next day.

With direct sun it jumps to 100% within an hour. This can't be right. I'm suspecting the controller is not reporting accurately. Rich Solar customer support is totally worthless. Thoughts?

 

[ianganderton]

Exactly what batteries have you got and how long have you had them?

What fridge, tv and fan are they? Do you know how much energy (watts or even better watt hours) they draw?

 

[bobbyq]

4 Duracell golf batteries. Tiny fan, energy star half height fridge and 32 inch TV.

My question isn't about the load, as the fridge, tv and fan from sunset till morning only use 5% to 10% max. I'm confused that when I tested it multiple times by unplugging all loads and turning invertor off at 100% it drops dramatically as soon as sun isn't hotting panels. It seems it should maintain the charge with zero load.

Last night it had 70% at sundown, dropped to 60 pretty much immediately and was 55 when I woke.

 

[bobbyq]

It stays at 100 as long as sun hits it with the appliances and running power tools. Oh, 4 100watt panels. Everything is 24volt.

I don't want to expand till I work the bugs out.

 

[John Frum]

Do you have a shunt based battery monitor?
If not you should get one.
That way you can see if you have a phantom load
Make sure there is no way a load can bypass the shunt.

 

[John Frum]

The other good tool to have is an ac/dc clamp meter.

 

[RCinFLA]

Golf cart lead-acid batteries are vunerable to positive plate grid corrosion due to excessive float charging which raises internal resistance.

The grid is the metal structure to support the lead paste of the plate. Positive plates are bathed in oxygen gas bubbles during charging. The positive plate gets coated with lead-oxide which makes it the positive plate. Negative plate is lead and bubbles off it is hydrogen gas during charging.

Gas comes from splitting water in electrolyte causing electrolyte level to drop and becomes more concentrated sulfuric acid solution. The gas creation accelerates when batteries approach full charge

You can do a load test to verify. It will have a greater voltage drop with load current. Capacity is still there, just cannot really use it because of excessive series internal resistance. No way to fix it once corrosion occurs. It is the second most common reason for lead acid battery failure, after sulfidation from lack of timely recharging resulting in hardened lead-sulfate crystals on plates which reduces available capacity.

Another less common issue is stratification of electrolyte acid concentation. Normally the discharging and recharging stirs up the electrolyte to keeping it from separating with higher acid concentration dropping to lower section of battery. This can be fixed by some discharging and recharging if has not been going on too long causing damage to lower section of plates due to its over-discharging over time. A battery failure due to stratification will have holes eaten in lower section of plates because upper section of plate with diluted electrolyte has not been providing any of discharge capacity.

 

[tictag]

I would agree with your suspicions, if the battery is actually powering your loads overnight with very little reported change in SoC i.e. 65% to 60%, this doesn't sound like a capacity problem. It sounds to me like your SCC is reporting SoC based on voltage (which is inherently inaccurate anyway) and, furthermore, it is considering something like 13.8V or higher as fully charged i.e. 100%. As soon as a charge voltage is removed i.e. at night, your battery will naturally settle down to its nominal voltage, which is around 12.6V for a fully charged lead-acid. Your SCC might be interpreting this as 65% SoC.

If these are SLA, you could buy a cheap hydrometer to accurately determine the battery's SoC or, better still in my humble opinion, get yourself a proper battery monitor like the Victron BMV or similar. These calculate SoC based on coulomb count, not voltage, and furthermore account for the puekert effect, which most cheaper battery monitors do not.

In terms of fixing the issue, if you unit is still under warranty I'd go that route. If not, are you able to configure the SCC? Might just need tweeking?

 

[Johnny16]

4 Duracell golf batteries. Tiny fan, energy star half height fridge and 32 inch TV.

My question isn't about the load, as the fridge, tv and fan from sunset till morning only use 5% to 10% max. I'm confused that when I tested it multiple times by unplugging all loads and turning invertor off at 100% it drops dramatically as soon as sun isn't hotting panels. It seems it should maintain the charge with zero load.

Last night it had 70% at sundown, dropped to 60 pretty much immediately and was 55 when I woke.

I have 8 new Duracel GC2 230ah batteries and have similar results. I have 2,000 watts of solar panels connected. Batteries are fully recharged by noon with 28.8v and float charged for the rest of the day. Once night time with no solar, voltage is at 25.7v. Drawing 400w for a few minutes drops voltage to 25v.

I originally bought 4 GC2 batteries but got 4 more because voltage dropped was more worrisome.

After sunset, overnight power draw of 70ah to 80ah leaves morning voltage at 24.2 to 24.5.

8 of these batteries gives me 460ah at 24v. My power draw is less than 20amp at any given time so i should get stated amp hour of 20 hour discharge rating.

In conclusion, these batteries performs poorly.