Controller and two batteries question

[Muddyrascal2020]

Hi everyone new to the site and wanted to pick your brains to confirm I’m doing the right thing with me e bike. I have a specialised epic with 1000w kit I fitted. I run two batteries for extra range. But I want to use a higher amp controller. Mine is currently only 20amp max.
it my understanding that if I got a higher amp controller let’s say 40amp then I am still limited by my discharge limit of my bms on the battery.
am I right by thinking that if I have two batteries both with seperate bms that allow 20amp then my controller would receive 40amps ??

 

[Nealh]

If the batteries are wired in parallel then theoretically yes, if they discharge equally.

 

[Andy-Mat]

If the batteries are wired in parallel then theoretically yes, if they discharge equally.

But they never do!
These Li-ion Batteries need to be most carefully handled. Between two batteries as the Op is talking about, is basically a specially designed BMS also needed!
That bike fire on YouTube is according to a friend of mine, an attempt by the biker to do just that. You can even see that he has two batteries if you watch the video carefully, one on the downtube and one at the rear where a rack would go.....!

It is far too dangerous for an untrained person (I include all electricians in that comment!) to "play around" with e-bike batteries. The results can be life threatening!
To fully put out the burning bike in the video, the whole bike must be completely submerged in water for an hour.
Andy

 

[Nealh]

My ones when paralleled discharge pretty much inline to within 0.03v even those of differing capacity.
My Swizzbee is hungry being an old brushed motor system(it can often demand 30a constant) has a 23.2ah and 17.5ah which are always used in P they act as one and voltage declines quite uniformly, batteries must be of the same nominal voltage and must be charged to the same voltage or within 0.025v before paralleling.
Charging though has to be carried out separately.

No use paralleling a tired battery with a good new one ideally they need to be similar age.

 

[Andy-Mat]

My ones when paralleled discharge pretty much inline to within 0.03v even those of differing capacity.
My Swizzbee is hungry being an old brushed motor system(it can often demand 30a constant) has a 23.2ah and 17.5ah which are always used in P they act as one and voltage declines quite uniformly batteries must be of the same nominal voltage and must be charged to the same voltage or within 0.025v before paralleling.
Charging though has to be carried out separately.

No use paralleling a tired battery with a good new one ideally they need to be similar age.

We are both putting it simply and of course you are fully right.
The dangers from these batteries for "newbies" are huge....
regards
Andy

 

[Nealh]

Yes for the uneducated they are or can be dangerous.
Those who have issues tend to be careless or not careful enough about being careful.
The extreme cases of fire is down to abusing them beyond their capabilities with high power controller/motor combos for the battery output or self builds without added safety within the build design to save a few $$'s or ££'s.
One does have to rigidly stick to a protocol of charging and connecting, I have wired externally to my packs/batteries a separate cheap £2 or £3 led/lcd display meter for checking each battery voltage, it uses a % reading, a voltage reading and uses a segment bar reading.

 

[Muddyrascal2020]

Thanks for the info and the warning of safety. So may be best if I tell you exactly what I’m planning and see if more experienced e bikers can tell me if I’m going the right way.
so 1000w 48v hub motor
2x 48v 15ah batteries with seperate a bms rated to a discharge rates of at least 20amps each bms.
then run this through a 35-45amp controller, Im referring to the max amp rating here.
im hoping to still have around a 25 mile range I currently run a 10ah and 12ah together and can do about 30+ miles on my 20amp controller .
I’m trying to achieve getting a little more torque and retain simular millage.
what’s your thoughts?

 

[Muddyrascal2020]

Oh and forgot to say obviously I will charge them separately as I do with my current set up

 

[Nealh]

I would have thought 30/35a will be more then ample for a pair of batteries rated at 20a continuous in Parallel, knowing which actual cells are used would give more piece of mind. Not knowing just the brand but the exact cell and it current rating etc,etc. The critical bit is to use a meter to verify voltage is within 0.025v before paralleling them.
In use should a battery become disconnected for any reason don't reconnect the Parallel connection as likely the voltage will be quite different, the heat transfer and the speed (milliseconds) from one to another will be /can be dangerous with many 100's of amps flowing. The heat transfer can damage wiring, pcbs, bms and cells.

For real world paralleling each battery should be rated and able to supply the controller current individually should there be a disconnect issue, they then can be used singularly rather then risk an issue occurring.

With my Swizzbee both batteries are paralleled both have a an 80a output rating or higher so both can comfortably work as a single power source.
The same goes for all my batteries they all can cope with my BMS's max continuous output singularly if need be.

 

[Muddyrascal2020]

Ok so this might sound like a silly question but if my battery amps are higher than the controller is rated will the controller just limit the amps or will is cut power or just burn out the controller.
im asking as I may buy a 35 or 38 amp max controller but the combined batteries could in theory give 40amps or mor depending on bms discharge limit.

 

[Benjahmin]

The battery current limit is what the battery is capable of delivering without damage to the battery. The actual draw is determined by the controller, so 38A controller will allow 38A from battery. Two batteries each capble of 20A will allow 40A but you should, if possible, have battery max at 1.5 times the max controller current, to avoid voltage sag.
The bms max is different to the battery possible max current. Each cell has a maximum current it can give. The battery maximum current is calculated by multiplying the cell current by the number of strings in parallel within the battery. This may then be 'choked' by the bms, so avoiding cell overheating.

So your proposed controller is capable of delivering 38A. Therefore your combined batteries should, ideally, be capable of delivering 57A.
Jeez, that's one humungous battery/motor combo (it weighs HOWMUCH?!!). At that weight I think I'd be looking at a motorbike.

p.s. I hope you've got the mother of all braking systems.

 

[Andy-Mat]

The battery current limit is what the battery is capable of delivering without damage to the battery. The actual draw is determined by the controller, so 38A controller will allow 38A from battery. Two batteries each capble of 20A will allow 40A but you should, if possible, have battery max at 1.5 times the max controller current, to avoid voltage sag.
The bms max is different to the battery possible max current. Each cell has a maximum current it can give. The battery maximum current is calculated by multiplying the cell current by the number of strings in parallel within the battery. This may then be 'choked' by the bms, so avoiding cell overheating.

So your proposed controller is capable of delivering 38A. Therefore your combined batteries should, ideally, be capable of delivering 57A.
Jeez, that's one humungous battery/motor combo (it weighs HOWMUCH?!!). At that weight I think I'd be looking at a motorbike.

p.s. I hope you've got the mother of all braking systems.

Well put!!
Andy

 

[Muddyrascal2020]

Brilliant great info no idea what the battery rated amps are on these batteries all I know is they gave Samsung 18650 cells they are 48v and around 15ah.
Iv got a much better understanding now So thank you.
yes she is a bit chubby I keep the batteries as low and fat forward as I can to distribute the weight as best I can but being a full suspention bike it handles the bumps fairly well. Iv got motorbikes too but enjoy exploring the footpaths on the e bike and use it as a computer had the same setup for around 5 years now and done nearly 20,000 miles on it been super reliable.