Adding New Battries

[Off grid in Tonopah]

    I'd like to pose a question to the board for some insight and advice. I currently have a 660 amp/hour, 24 volt battery bank composed of 3 strings of 4 T105's. I hit these batteries pretty hard cycling them 30% to 50% daily. I recharge daily to 100% using my generator and Outback inverter, and I equalize once a month. The battery bank is now 1 year old and I'd like to add some more capacity. This would be adding 4 more T105's giving me 4 parallel strings.

    First question: Knowing the down side of too many strings of batteries but not being able to add capacity without doing so, I'll have to live with it and watch my voltages.  So would it be best to add the string of four new batteries as a whole new string or better to place one new battery in each of the four strings? Logic tells me that to keep the string voltages as equal as possible I should spread out the new batteries. But some validation of that logic would be nice.

    Second question: From reading up on mixing old and new batteries it seems the down side is that the new batteries will drop to the performance level of the older ones and loss the "Newness" is the biggest hit. Is that correct? Or is there potential damage to the existing batteries in the bank from having to operate with newer batteries in the system?

                                         Thanks for any input

                                         Bob

[Flux]

I am sure you will get so many conflicting answers that you will be no wiser at the end of this.

I have personally not been bothered much about parallel strings. If you can avoid trouble with series strings then you should be ok.  To me that absolutely means not mixing old and new batteries in a series string.

As long as all the strings are ok and you can reach the full charging voltage regularly the I don't see any serious problem.

There could be an issue mixing different types of battery in parallel ( I wouldn't mix VRLA with flooded) but other than that I don't see any problem,I don't even see that you need degrade the new cells as long as there are no duff ones in the old string. If the old ones can't be got up to full charging volts for the new ones you may have an issue but if they are still good and working ok just carry on.

We don't live in a perfect world just carry on and do the best you can. If these were not being regularly charged then the issue would possibly be a little different.

Flux

[wpowokal]

Bob everything Flux said and I use nine paralell banks of 660A/h batterys.

They do take charge at slightly different rates which says they are discharging likewise, but then I forget about it and let them get on with their job.

allan down under

[Adrian L]

I'm a bit of a information nut, and read the net for hours and hours on everything renewable energy related...I have constantly read over and over that mixing battery types and battery ages is a Sin....

I have had a small solar power system for 7 years now.  I have a Sealed lead acid bank totalling 24v 54amp, and a Lead acid bank totalling 24v 65 amp, The SLA bank is about 5 years old and the lead acid 7 years.  I have run them in parallel strings without issues.  They get daily use and occasional extemely heavy use 2000 watt plus draw.  They are holding up exceptionally well and maintain 80% of their rated capacity after all this time. The charging efficiency also rates fairly well according to my Plasmatronics (amp logging) regulator.

These are run of the mill deep cycle batteries, and chinese made SLA's.

As per the above comments, when you get into trouble is if you mix old batteries in series strings with new batteries (they have different resistances) - mind you this can happen to batteries of the same ages as well.  It's always good to occasionaly parallel all your batteries then give them a full charge at 12 volts to bring them back to a similar charge level / resistance, or do each one individually, obivously this is not always practical, but recommended.

[Volvo farmer]

Flux nailed it.

I know nothing about lots of parallel strings other than what I've read. It seems to me that it makes sense to avoid many parallel strings, if it is possible in the design phase, economically.

In the end batteries come down to a dollar/kwhr number, plus the hassle factor of changing them out. I was reading my Trimetric manual the other day and he was talking about how deeply batteries should be discharged and how long they last and said: "This is really just another way of saying that battery life is related to the amount of amp hours removed."

This statement opened my eyes somewhat. I took it to mean that you could discharge a 1000Ahr battery bank 25% to 750Ahr daily or you could discharge a 500Ahr bank to 250 Ahrs (50%) daily, but in the long run, a battery wears out at a certain "lifetime Ahr" number, and the $ figure would end up mostly the same in the two scenarios, as long as daily recharging is taking place.

[Off grid in Tonopah]

    Thanks for the feed back guys. I'll rest a little easier when adding the new storage that I'm not throwing good money at a capacity problem and only wasting it or worse making non problem into one.

                              Bob

[Wheeler]

This is some very good information, it gives me some food for thought as I will need to reconfigure my battery bank in the near future.

[kitestrings]

Hi Bob,

We faced a similar situation with our system some years back.  The existng bank (purchased with our initial system) simply wasn't big enough, and buying all new batteries just was not in the budget at the time.  Plus, it just seemd like a waste to me.  In our case the first batteries suffered more from inactivity then old age, so we had to work on them a bit.  It is probably quite dated now, but a fellow named Michael Hackelman wrote some useful strategies on testing, caring for, and in some cases rescuing [partially] sulfated batteries.  Anyhow, I think it worked out okay overall.

In your case, at 1-year old, your batteries are relatively new and probably just hitting their stride.  Generally they're a little stiff when new, so I wouldn't hesitate to add another string  - of the same make/capacity is even better still - to your bank.  Just make sure the connecting cables are adequately sized.

Most manufacturers will tout their (improved, superior) plate construction, DOD rates and life expectancy under very controlled conditions, and rarely are real-life conditions ideal.  Having said that, lead-acids are pretty resilient.

We're now replacing our existing bank, after almost ten years.  The truck is scheduled to be here friday with the new ones.  We'll have two parallel strings (48V), and I'm giving serious consideration to replacing a single series-string at their ~mid-life.  The alternative is very high cost at end of life, and dismal performance in the final stages.

One final thought.  We've have, and have used, both the T105's and the L16HC's.  I ran some cost comparions based on $/AH nsome years ago.  The T105's actually came out on top as I recall, but the life expectancy is much lower (4-7 vs 7-11 years).  We had good performance and reliability with both.

kind regards,

-kitestrings