Old and New lead acid battery with constant load

Re: Old and New lead acid battery with constant load

Welcome to the Forum Phmadeira!

phmadeira wrote: »

Hello
I know everybody say that it is not good to add new lead acid battery to an old battery bank.

The old/new battery mix is not a Fail/Not Fail type condition... It is a mix of issues that depend on how you wire everything up, maintenance, and your expectations.

But it is still true with a constant load ?
I do have my freezer always took 2.5 amp from the bank
in this case i think that the battery cannot interact together?

Lets say you have 4x 12 volt batteries in parallel and they are a few years old.

You add 4x new 12 volt batteries in parallel with the first ones. The new batteries will (probably) have higher specific gravity which means higher output voltage (by a little bit). What can happen is the new batteries tend to supply more of the current during discharging (a bit higher voltage and lower electrical resistance)... More or less, the new set can carry more of the discharging/charging current while the others only supply a smaller part of the load support. The newer batteries may tend to "age" faster than the older set.

If you have a 12 volt battery and put another 12 volt battery in series for 24 volts--The two batteries will probably have different "capacity" between the two. The older battery may have 20% less capacity and as you discharge the two--The older battery will "go dead" first--And will quickly die there after (if you deeply cycle your battery bank).

Also, older batteries tend to "self discharge" more than newer batteries--So you will have to "equalize" the string more often. Equalization will be a bit "hard" on the new battery as it has less self discharge (equalization is "over charging" one battery to get current into the other battery that is not 100% charged).

So--In the end, adding batteries to and existing string can cause the new batteries to not last as long--Plus it puts you into a situation where you are replacing a battery now, another a few months down the road, etc... Or in constant maintenance of your battery bank.

It certainly can be done, and if you have an existing bank where one battery "went bad"--You probably would prefer to replace the existing battery because the rest of the bank still have a number of years left of life... But when you are mixing/matching--It will require more work on your side to keep things running well.

If you are in the US (or have friends here), I would suggest a DC Current Clamp meter like this one (really an AC/DC current clamp+multi meter for ~$60 USD). It makes it much easier and safer to measure the shared current between your battery strings and solar panels--And find bad connections/failed battery cells/etc.

if i switch off my freezer i see that my old bank 450 amp/h is charging my new bank 200 amp/h with 0.5 amp.

Resting Battery bank Voltage does represent (roughly) the specific gravity of the batteries (higher SG, the more fully charged a battery/cell is, the higher the resting voltage of the bank is).

If your old battery bank is "charging" the new bank and the batteries are the same model/type/chemistry--Then that would suggest that the old bank is at a higher state of charge than the new battery bank (could be a "surface charge" on the old bank, temperature differences between banks, etc. too).

I had a battery monitor for the Both bank old and new who are in parallel , and one battery monitor on the new bank. Then i put back the freezer, i get in Total minus 2.5 amp from the two bank , and minus 1 amp from the new bank that mean minus 1.5 old bank and 1 amp new bank.

Well--That is not bad--If all things were equal, the 400 AH battery bank would carry 2/3'rds of the current and the 200 AH battery bank would carry 1/3'rd of the current (both discharging and charging).

SO what to you think about mixing old and new battery in this case with a constant load.

Since you already have the two banks, and can measure the current to each string during charging and discharging--Why not. Just monitor the voltage and current of each cell/battery and watch the current (charging/discharging). If you see things "diverge"--Then start looking for reasons.

You should also get a Hydrometer so you can measure specific gravity of each cell (say once a month, and you can monitor a "pilot" cell every day or so to ensure your recharging the batteries back to 90% or more State of Charge at least several times a week. Try not to let them discharge below 50% state of charge very often (and quickly recharge). And never allow them to go below 20% State of Charge or "go dead"--That usually will destroy your battery bank/specific cells. Deep cycling also reduces battery life.

Please read about batteries from these two FAQs... The better you can take care of the them--the longer they will last.

Battery FAQ
www.batteryfaq.org

Good Luck!
-Bill