Hello Everybody!

I have an off-grid photo-voltaic system in which I'm using 2 lead-acid batteries with nominal voltage of 12V and rated capacity of 150Ah (each) to store the energy. The two batteries are connected in series, resulting in a 24V, 150Ah (3.6 kWh) battery bank.

I am thinking of extending the bank to double the storage capacity. I know that adding new batteries to old ones is generally a bad idea, but mine is a special case, hear me out.

I would like to add two more solar lead-acid batteries which are absolutely identical with the ones that I already have (same manufacturer, same model, same capacity). The idea is to connect the two new batteries in series (just the new ones), to obtain a second pair, a second 24V unit made of 2 12V batteries. Finally I would connect the old 24V string with the new 24V string in parallel and the end result would be a 24V, 300Ah battery bank (7.2 kWh in total). Once more: old batteries in series, new batteries in series, old pair with new pair in parallel.

Now, I know that even though the specifications of the old and new batteries are identical, they are not identical in reality because they are from a different batch and the old ones have already lost some of their initial capacity. BUT I have only been using the old pair for 6 months and I have been kind to them (never drained more then 50% out of them, in fact most of the times I only drained about 10-30% out of them). What I'm wondering about is if those 6 months of kind use could be overlooked and if I could combine the old pair with the new pair without problems.

The batteries are sealed batteries designed for photo-voltaic applications. It is not clear from the specs if they are AGM or gel, they seem to be some kind of hybrid between AGM and gel, with AGM used as separator and some kind of solar "nanogel" used as electrolyte.

There is one important thing to mention. I never could charge these batteries according to the specifications of the manufacturer. the manufacturer recommends an absorption voltage of 14.4-14.8V and a float voltage of 13.8V. But for me that did not work, the batteries were makig strange bubbling/fizzing/boiling sounds, so I was always charging them with 13.8V absorption voltage and 13.5V float voltage. Of course I will never know if this could have damaged them (sulfating them), but one thing is sure, this way they don't boil/bubble/fizz.

So what do you think? Could I risk adding the new pair to the old pair in parallel?

Thank you!

 

With only 200 or so light discharge cycles, those old batteries may actually now have slightly greater capacity than the news do. They gain Ah capacity after a short break in period, and then it slowly tapers off over time.
1000 or more charge discharge cycles is an average life span of well treated deep cycle batteries.
Check voltage carefully on your 24 volt pairs before you parallel them. Try to have them exactly the same voltage or with .1 volt at most.

 

I suggest you replace the old batteries with the new batteries. After the new batteries have gone through at least 50 dis-charge - charge cycles then add the old batteries back in.

The absorption voltage you called out (14.4-14.8) is right for flooded lead acid batteries but is too high for AGM or Gel batteries. I suggest you contact the manufacturer for the correct charging profile.

 

Thank you for your answers!

Yes, I will make sure to have the exact same voltage on all 4 batteries before putting them in parallel. Or to have a difference of at most 0.01V.
Working the new batteries for 50 cycles before putting them to work is actually a good idea, thanks!

I did try to contact the manufacturer about the charging profile, they redirected me to the local supplier who sold me the batteries. Those guys don't have a clue about these batteries because they mostly only sell car batteries. So they have just replaced my batteries, but those work the same, of course, same bubbling/fizzling/boiling at higher voltages.

 

What is the brand and model number of the batteries?

 

Manufacturer: BSB Power (http://www.bsbpower.com/)
Model: Solar 12-150
Instruction manual: http://www.e-acumulatori.ro/files/files/detalii-tehnice-116.pdf

 

OK, I guess they know their own batteries. You might read the Wikipedia page on them. https://en.wikipedia.org/wiki/VRLA_battery

 

Yepp... one thing I never figured out weather that fizzing sound is normal and gas recombination solves it or it's actually loosing gasses and capacity.

 

Once you have reached a current density high enough to allow hydrolysis of the water to begin you are wasting input power making gasses. A point close to, but below that current is where the chemical conversion of the lead compounds and the sulfuric acid is at its peak. Most input power going to the chemical recharging process and very little to other non productive reactions. At lower current levels your heat energy losses are lower from I (squared) R as well.

Short: slow recharge wastes less input energy.

 

With off grid power systems you have to charge them when you are generating power (sunlight). You have just so many hours per day to do it.

 

Kermit2, I think Lestrevaled is right. when the sun in shining, I want to push as much energy into the batteries as possible. I don't care if for every unit of energy that goes into the batteries, another two units are wasted, because if it doesn't go into the batteries, it's wasted anyway. What I do care about is not to damage the batteries and not to reduce their capacity. So if they bubble but the gasses are recombined (not lost), that's OK. But if they bubble and the gasses escape, that's bad. This is what I'm wondering about. But I don't know what that bubbleing actually means: losses of gas or no losses of gas.