Batteries in parallel of different age?

If I hook up 2 batteries in parallel and increase the current the voltage stays the same. Would the load still pull only the current it demands? In theory I could hook any number of batters in parallel and the load would just run longer.

If batteries of same make and voltage, but of different age. Say the batteries have a life expectances of 500 charges/discharges. And I have a battery that has 250 charges/discharges cycles on it and one that is new. When hooked in parallel would this cause any problems to the batteries or load?
And what a fully charged battery was in parallel with one of half charge?

Jay Fox said:

If I hook up 2 batteries in parallel and increase the current the voltage stays the same. Would the load still pull only the current it demands? In theory I could hook any number of batters in parallel and the load would just run longer.

Correct

If batteries of same make and voltage, but of different age. Say the batteries have a life expectances of 500 charges/discharges. And I have a battery that has 250 charges/discharges cycles on it and one that is new. When hooked in parallel would this cause any problems to the batteries or load?

The older battery will not carry it's share of the load so it eventially will be as if it is not even hooked up, just along for the free ride
And what a fully charged battery was in parallel with one of half charge?

The batteries will tend to balance themselves by allowing current to move from one cell to the other until they equalize the terminal voltage.

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Charging parallel connected batteries is not the best way to go unless special battery charge equalization steps are taken.

Lefty

Are you talking about batteries or cells? Same or different chemistries?

Battery packs should be made up with cells that are as similar as possible and always charged and discharged together, never seperatly otherwise cell imballance occurs which means you get current flowing between stronger and weaker batteries (wasted as heat). If this is severe enough one cell can actually 'invert' which basicaly means it's ruined for energy storage, they waste a lot of energy as heat and can be a fire hazzard. If batteries in parallel are taken from the same batch (preferably immediatly next to each other on the production line) and run in parallel they can be essentially treated as a single 'cell' for all intents and purposes.

There's nothing special about equalization though, it's simply a controlled amount of over charging following a normal charging cycle which allows weak cells to catch up to stronger cells. This is done in virtually every cheap cordless phone in existence, as they typicaly use a 3 cell NiMH or NiCad battery pack with a constant trickle charge of 1/10th capacity.

Inteligent charging schemes and specially tapped battery packs to allow for voltage monitoring are needed to really get the last bit of useable energy storage out of a battery.

If you're going to parallel batteries, make sure they are all EXACTLY the same, and all brand new - you don't want to mix old and new batteries.

You all are awesome my project is with an E-Bike it has a 36 volt 8amp hour sealed AGM Lead Acid and not sure how many cells. The batteries can be bought individually and installed into the built in charger assembly of 115vac input and a 36 vdc 1.6 amps output. The battery and charger assembly can then be pulled out together and carried to the power outlet. Or more in my case I can easily bring the out let to the bike. Where I want to hook 2 batteries to the same charger. One battery would be mounted to the bike. It looks as if I have some more homework to do. First off the batteries ARE the same but of different age and different production line times. I am learning more about charge stabilizations. Thank you for all your help.

You should use an isolator circuit between the batteries when charging them. Any good bank of batteries employs a charging isolator just as dual batteries in automotive situations does.

Most electric fork lifts and pallet jacks I've seen all charge the entire pack without issolators. They don't usually go higher than 24 volts though.

Hi, parallel battery charging always opens a can of worms. Just look at the threads on 4 wheel drive forums regarding dual battery setups!
In Australia they make a car (3.0 TD Holden Jackaroo) which has two main cranking batteries wired in parallel and charged together. I believe Toyota also has a twin setup for very cold climates. Admittedly, they are the same type, age etc. However, over a period of time differences in the batteries will develop due to manufacturing differences.
On these forums there are people with simple setups such as a manual change over switches to very sophisticated electronic solid state relay cut-outs etc. They ALL seem to work. The most common setup seems to be by isolating diodes.
RH

Isolating diodes in an isolator circuit between the batteries look like my next project. I will keep you posted on my progress and if I have any more questions. I'll see if I can stuff any of those worm back in that can before HiTeck has to learn me a good one.

Just don't forget, diode drop is wasted power.

I think that the drop in power across the diodes would be an acceptable loss in this case. Unless you foresee a large resistance that would stall the large current draw when in full load? During the charge it would be plugged in. And have read some were that it takes .06 cents to charge. I can go 20 MPH for one hour with out peddling the bike. I find that I don’t use the motor going down hill and can peddle it about 30 MPH. And on the way up I peddle with the motor and we go about 20 mph up just about any hill.
Now I do have 2 batters to hook in parallel and the batters are a series of cells in series. So then it would truly be a series parallel circuit.
Thank you Sceadwian