Lithium batteries

The "safe" answer is no !

theoldwizard1 wrote:
The "safe" answer is no !

That is indeed the safe answer however....

I've seen lots and lots of posts about batteries especially different size batteries that are 100% ... ****. no other way to put it.

Good chance your 200 amp battery is two 100 or 4 50s in the same box.
(Not 100% by any means but I have seen it).

The Myth "Well as the batteries run down the 100 amp will run out while the 200 is still half charged".. BULL, total BULL

Though LiFePO4 do not drop voltage like Lead Acid do as they run down (Till they hit empty then they do the rock in freefall thing) they do... very very slowly.. drop voltage as they discharge.

And the battey with the greater state of charge (Voltage) Will supply the power UNTIL it's at the same SOC as the other battery.> Then they will "Lock" together SOC wise..

Same with charging.. If one is at a higher state of charge it will not accept much till the other one catches up. then they "Lock"

HOWEVER in the case of different LiFePo4.. there is the Battery Management System... How well they are designed may affect the results.

I plan to put in 3 100 amp's some day (This is planned for next year) to replace 300 amp hours of Lead Acid.. But I got to pay for Christmas first (Actually it's paid for but I blew my cushion).

No.

While you can do it safely it is not best practise, nor recommended.

The batteries should be wired in a balanced manner, be the same number of amp hours each and the same voltage.

Yes.

Depends partly on type of batteries, and also how are they connected?

Too many questions to give you a good answer.

wa8yxm wrote:

Good chance your 200 amp battery is two 100 or 4 50s in the same box.
(Not 100% by any means but I have seen it).

The Myth "Well as the batteries run down the 100 amp will run out while the 200 is still half charged".. BULL, total BULL

Though LiFePO4 do not drop voltage like Lead Acid do as they run down (Till they hit empty then they do the rock in freefall thing) they do... very very slowly.. drop voltage as they discharge.

ahh but they do drop , just not to the same extent as a lead asid. so when I am at 100% charged on my LiFeP04 I am at 13.6V when I am at 10% I am at 12.0V so still quite a range. and yes a 200 amp battery is made op from smaller chunks but not two 100 amp batteries. it may be made up of 100 cells (like battle born uses) and one BMS to combine them into 1 battery. or in the case of prismatic cells you would put two 100 amp batteries into one box keeping them like they are, you would remove both 4s BMS cards from them and use one 8sBMS to combine the 8 cells into one battery.

you should keep them the same so it ballances the amprage between each battery. if you use the situation above with one 100 amp battery and one 200 amp battery in parralell and you put a 100 amp load on it, yes the battery voltage will drop the same but the capacity of the 100amp battery will drop twice as fast if you have a 50amp BMS on each battery (for example) so once the 100amp battery gets down to the cut off voltage (10% in my battery) it will shut off trying to throw all the load to the other battery but since that load is twice the amount alowed by the second battery the BMS will shut it off on protection also. so in essence you can only use 1/2 the capacity of the bigger battery. by keeping them the same you can use full capacity (or what you want to conside full capacity. mine is 10% to 90% when I am camping with a run to 100% ever so often to balance the cells.)

if your amp load is small enough to be handled by one BMS then it is cheeper to vbuild one big battery using more cells, for example I have a 320 ah (314 actual capacity) in my camper made from 4 prismatic cells and one 150 Amp BMS. I can move that battery to my 5th wheel and have more power available then my four 6V GC batteries give me by 100 Ah but the BMS wont give me enough amprage to run the furnace and microwave at the same time for instnce. so I can add another 320 ah battery with another 150 AH BMS. but if I just wanted to say run the furnca because I am winter camping so I want that 600AH of power, I could just take the two 4s BMS off and use a 100amp 8s BMS that turns all 8 cells into one battery instead of two in parrelell. thats the great thing about using cells and building your own batteries you can configure it how you want it. tome a single battery will always be better, but it get expensive when your looking at 8s/12s/16s BMS units with high amp capabilities which is why it can be quite a bit cheaper making two or three smaller batteries.

As long as the voltage is the same. you can

wa8yxm wrote:

Good chance your 200 amp battery is two 100 or 4 50s in the same box.

12V lithium batteries are typically made up of 4 3.4V cells (lead acid are made of 6 2.2V cells). Larger capacity, in both cases, means larger cells.

In parallel and same voltage (IE both 12V) yes, not an immediate issue per se.
Not an expert on LiFePO4 batteries, but the long term issue of connecting batteries of different capacities or different age is that they will discharge and recharge at different rates. Yes they will always seek to equalize to the same SOC % when being drawn upon or unused, but when charging, I believe you are potentially or realistically over charging the smaller or older battery while charging.
That said, I've seen and used alot of mis matched batteries in parallel operations. Does it work? Yup, just fine. Is it shortening the life of one or more of the batteries prematurely? Probably

But I can say with matched batteries, hooked in parallel only for charging, but separated for loads (boat, house and start batteries) with a single bank charger and decidedly different loads, those batteries have lasted as long or longer than they should. But they're only used around 20 days a year.

StirCrazy wrote:
you should keep them the same so it ballances the amprage between each battery. if you use the situation above with one 100 amp battery and one 200 amp battery in parralell and you put a 100 amp load on it, yes the battery voltage will drop the same but the capacity of the 100amp battery will drop twice as fast if you have a 50amp BMS on each battery (for example) so once the 100amp battery gets down to the cut off voltage (10% in my battery) it will shut off trying to throw all the load to the other battery but since that load is twice the amount alowed by the second battery the BMS will shut it off on protection also.

Again I call BULL. You are assuming that each battery provides 1/2 the load current. This is beyond belief, it very simply WILL NOT HAPPEN in fact IT CAN NOT HAPPEN. for as you yourself admitted the voltage drops as the SOC goes down. Thus the if you put a 100 amp load on the pair the 100 amp battery will not provide 50 amps but 33.33333333 and the 200 amp will provided 66.6666666d The current divided PRECISELY according to the capacity of the batteries.

This is the ONLY way it can happen. for the battery with the higher state of Charge will provide ALL the current till the SOC's match (Or nearly all)

This is the very MYTH that I warned about in my earlier post.

Now instead of electricity think water

you have two tanks side by side
one tank holds 1,000 gallons and the other thank which is the same height and the same depth but TWICE as wide. holds 2,000 gallons.

They are connected at the bottom by a "T"

When you draw water off the "T" which one is going to hit empty first?

Now think about this... if the water is HIGHER in one tank, that tank pushes harder and will provide all the water till the levels are the same. then they split the load 1/3 2/3

It is exactly how the batteries work.

The only warning is the charger

If you set up a charger for 300 amp hours and one of the batteies goes HIGH RESISTANCE... I'm not sure what happens (Due to the BMS systems. they may protect or not)