wa8yxm
Davison Michigan (East of Flint)
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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)
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time2roll
Southern California
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wa8yxm wrote: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 closer to true for lead-acid due to the high internal resistance. LFP will be initially closer to 50/50, but the truth is even if they are identical 200 amp batteries in parallel the current will not be equal due to variations in the cabling resistance etc. Even the slightest minutia of difference creates an imbalance.
One will generally discharge faster initially and as the actual voltage slowly differentiates the batteries will be closer to proportional. The battery that provides proportionally less at the start will provide more toward the finish.
Yes one of the batteries will inevitably work harder than the other at less than full cycling. And there are so many more cycles available it could be a decade before it is noticed.
For reference I have 2x 600 ah batteries in parallel and it is very common to initially see 60 amps from one and 70 amps from the other running the microwave.
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wa8yxm
Davison Michigan (East of Flint)
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time2roll wrote:For reference I have 2x 600 ah batteries in parallel and it is very common to initially see 60 amps from one and 70 amps from the other running the microwave.
Yes at those currents minor differences in cable resistance or internal resistance due to age WILL show up as differences in currents.
But by your own admission the batteries work out their differences over time..
Which is exactly what I said.
I would not fear parallelling two different size batteries so long as they are otherwise the same (Same chemistry, voltlages and the like)
Paralleling a LiFePo4 with a different TYPE of battery.. NOT a good idea.
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time2roll
Southern California
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^^^^ Yes. Agreed 100%
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StirCrazy
Kamloops, BC, Canada
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Joined: 07/16/2003
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wa8yxm wrote: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)
so i'll call bull on your atempt to bring fluid dynamice into this, voltage = presure or potential not capacity which would be the volume. but any ways..
I have done the exparament I mentioned above with a 80AH pack and my 320AH pack and it worked like I said, but thinking about it now that was under high draw , and they did share the load pretty much 50/50 until one couldn't, thats where my answer came from. and this could have been a function of the BMS also as the load was 90% of eaches capacity.
thinking about this now if you had a lower draw (under the BMS limit of both batteries) it makes sence that they could decrease the same as long as the load is able to be handled by both batteries on there own. maybe another exparament I can try this winter, once I get my reno projects done....
I myself still wouldn't paralel two different capacity batteries even if it did work as if I have to add another battery its because I want the capacity for a microwave or something else which will be high draw. I think for comercial batteries I would rather have three 100 AH than a 200 and a 100.
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S Davis
Western WA
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“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.”
Except you just made a 24 volt battery with a 8-S BMS. Eight cells in series makes 25.6 volts. To stay at 12 volts and up the ah you would have to use a 4-S BMS and use parallel sets of cells. The draw back is not being able to monitor each cell in the battery.
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jaycocreek
Idaho
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Even though it is possible,I chose not to parralel my 120ah battery with either of my other 100ah battles..
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StirCrazy
Kamloops, BC, Canada
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Joined: 07/16/2003
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S Davis wrote:“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.”
Except you just made a 24 volt battery with a 8-S BMS. Eight cells in series makes 25.6 volts. To stay at 12 volts and up the ah you would have to use a 4-S BMS and use parallel sets of cells. The draw back is not being able to monitor each cell in the battery.
true, you would be monitering groups of cells, but if you get quality cells that are matched, thats not an issue.
I still recomend building seperat batteries unless you want to go to a higher capacity and use a stepdown. but in my case I would do another 310AH battery and have two of them. so when I am using my camper I would just use one as thats 10 days camping with the furnace running 24/7, but for my 5th wheel I could have two in there and just move one back and forth between the 5th and the camper. one will give me more usable capacity than I have with my four 6V batteries so it will be fine when I am using the camper, but when I am going for a longer dry camping backwoods with the 5th wheel I can put the second in and double my capacity. being only 48lbs its not a big deal either.
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pianotuna
Regina, SK, Canada
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Joined: 12/18/2004
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wa8yxm and time2roll. I have great respect for both of you, but this time I have to disagree.
From Battle Born:
"Can I Connect 100 Amp-Hour Battle Born Batteries to the 270 Amp-Hour Battle Born Batteries?
Here at Battle Born Batteries, we offer two different lithium battery sizes: our 100 Amp-Hour BB100 series and GC2 series, and the new 270 Amp-hour batteries.
These two different sizes of batteries should never be connected. Any attempt to connect Battle Born Batteries of different amp-hours will result in a voided warranty.
You should only ever connect batteries of the same size."
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.
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wa8yxm
Davison Michigan (East of Flint)
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pianotuna wrote:wa8yxm and time2roll. I have great respect for both of you, but this time I have to disagree.
From Battle Born:
These two different sizes of batteries should never be connected. Any attempt to connect Battle Born Batteries of different amp-hours will result in a voided warranty.
You should only ever connect batteries of the same size."
Well naturally a company will use any excuse to void a warranty.
This is. however, good info.. For the very reason stated.. Voiding of warranty.
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