Parallel two 100ah: double the max draw?
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Bud Martin
Solar Wizard
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Yes, if all are equal, you can draw 100A from each battery so total draw will be 200A.
In parallel circuit current is additive, Voltage stays the same.
In series circuit Voltage is additive, current stays the same.
In parallel circuit current is additive, Voltage stays the same.
In series circuit Voltage is additive, current stays the same.
Which means, I would assume, separate fuse for each battery. Anything else?
If one battery goes down, the BMS of the other will cut the power as the load would then exceed 100 amps, please correct me if I'm wrong.
Thank you.
That’s about it, fuse or breaker on each.
you likely have adequate 100 amp wire size already from each battery to a busbar which is where the protection would go.
200 amp wire and protection from the busbar to load.
Wire size resource :
Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems
Engineering high quality marine electrical components for safety, reliability and performance
www.bluesea.com
What size fuse between each battery? 100Amp?That’s about it, fuse or breaker on each.
you likely have adequate 100 amp wire size already from each battery to a busbar which is where the protection would go.
200 amp wire and protection from the busbar to load.
Wire size resource :
Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems
Engineering high quality marine electrical components for safety, reliability and performance
Each battery fuse to match its size, thus 100 amp, and then a fuse to match the overall current going out of the battery array, in my case: 200 amp.
Got it thanks
chrisski
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I’m assuming these are lithium batteries, so i think these 100 amps fuse on each battery and the 200 amp fuse after paralleled are correct, but would need to check the spec sheet of the battery for max discharge rates. It probably is 100 amps or more so then 100 amp fuse is fine, but if not, need to fuse less.
i assume the ‘100 ah BMS’ has a max discharge of 100 amps, so this would be fine. Again, a spec sheet would tell.
Class T’s are recommended for lithiums. I have three class t fuses on my lithiums, one for each battery, and one once paralleled. Those class t fuse holders take up 6” each. They are not the small ATC blade fuses i had been used to dealing with.
If these are nor lithium, than IME the spec sheet is unlikely to state the max discharge rate, so that could be a mystery. On an prior set of lead acid batteries, I contacted my battery manufacturer to get the official answer.
i assume the ‘100 ah BMS’ has a max discharge of 100 amps, so this would be fine. Again, a spec sheet would tell.
Class T’s are recommended for lithiums. I have three class t fuses on my lithiums, one for each battery, and one once paralleled. Those class t fuse holders take up 6” each. They are not the small ATC blade fuses i had been used to dealing with.
If these are nor lithium, than IME the spec sheet is unlikely to state the max discharge rate, so that could be a mystery. On an prior set of lead acid batteries, I contacted my battery manufacturer to get the official answer.
Something to consider. A LiFePO₄ battery that states 100A max is likely claiming 100A of max continuous discharge current. It likely supports 200A for a few seconds to handle a surge. So putting a 100A fuse on such a battery probably isn't the best choice. Why have the fuse blow while the battery is perfectly content putting out 100A. If you want a fuse to blow before the BMS is forced to shut things down then you probably want a 150A fuse on each battery that is rated for 100A max continuous discharge. This allows the battery to be in its happy place all day without the fuse tripping. But the fuse will likely trip before the BMS shuts down to protect the battery from too high of a current.
My question would be then- why put a fuse out of each battery at all?
We have the overall fuse going into the bus (200 amp for example) and the BMS of each battery is definitely capable of maintaining the load for that battery.
Why do we need that extra fuse, then?
A BMS is no substitute for a proper fuse or breaker. Fuses protect wires, not components.
The MOSFETS in a BMS can, and do fail. In that case your wire becomes the fuse.
It then opens the circuit in spectacular fashion as it catches on fire ?.
Regarding post #11, if you use a 150 amp fuse be sure your wire is properly spec’d.
The MOSFETS in a BMS can, and do fail. In that case your wire becomes the fuse.
It then opens the circuit in spectacular fashion as it catches on fire ?.
Regarding post #11, if you use a 150 amp fuse be sure your wire is properly spec’d.
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Let's say you have a 2000W inverter and you have 2 12V batteries in parallel. The inverter can pull up to 200A from the battery bank. Each of the 2 batteries can provide 100A of continuous discharge current. When both batteries are working well there is no problem. The overall fuse (which should be 250A in this example) is there to protect the outgoing wiring incase a battery is shorted causing a massive surge.
Now let's say that one battery is having issues (perhaps imbalanced cells) and its BMS shuts down the battery. Suddenly, the 200A load will go to the one remaining battery. If you don't fuse each battery individually, that 2nd battery's BMS will shutdown quickly as it is being asked to supply 200A by itself.
So the debate is whether it's fine to let a BMS act as the only protector of the battery or if your system should be designed so the BMS never has to deal with such a situation. Some will say the BMS should only deal with the internals of the battery going wrong (such as imbalanced cells) and that fuses should be used to deal with too much current going to an otherwise healthy battery.
In my own system (2 12V LiFePO₄ batteries in series for 24V) I have a fuse to deal with over current. I have a battery protect to avoid over discharge. I have a temperature probe setup so my SCC shutdowns if the battery gets too cold. In my case the BMS is only used as a last line of defense for those situations and to deal with the individual cells.
Now let's say that one battery is having issues (perhaps imbalanced cells) and its BMS shuts down the battery. Suddenly, the 200A load will go to the one remaining battery. If you don't fuse each battery individually, that 2nd battery's BMS will shutdown quickly as it is being asked to supply 200A by itself.
So the debate is whether it's fine to let a BMS act as the only protector of the battery or if your system should be designed so the BMS never has to deal with such a situation. Some will say the BMS should only deal with the internals of the battery going wrong (such as imbalanced cells) and that fuses should be used to deal with too much current going to an otherwise healthy battery.
In my own system (2 12V LiFePO₄ batteries in series for 24V) I have a fuse to deal with over current. I have a battery protect to avoid over discharge. I have a temperature probe setup so my SCC shutdowns if the battery gets too cold. In my case the BMS is only used as a last line of defense for those situations and to deal with the individual cells.
Nobodybusiness
Collecting the leftovers of the Great Sky Reactor.
Problem is if you have a short either in the battery or the cable or BMS then all the current from the other batteries in parallel will dump their current to the short.
If you have it fused or a breaker at each battery it keeps that from happening.
chrisski
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I have cables that will help parallel a pack. I crimp lugs on those cables. I secure the cables o studs with nuts. The cables are the same size. My batteries are now properly paralleled.
defeedme
New Member
ok but what if we have two - new tech 230a batteries with both having their own 200ah advanced bms ? can you draw up to 200 still, even if one battery completely fails - do we still need to "wire and protect" for such a scenario? these are the 2 batteries in question which have some advanced features which may not need to have "wire and protection" as you state.. please advise.. https://www.amazon.com/Litime-LiFePO4-Battery-Lithium-Phosphate/dp/B0BYH8J176/
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