Adding batteries in series

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Starway

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Hi guys, 

‘I don’t know if anyone could help but I thought I would give it a shot. 

I have a ebike motor with a 60v controller that came with it but I have been using a 48v 15ah battery the whole time, the battery is actually 5 years old but I haven’t used it that much so it’s working great still. . I must say it’s working great  . I was thinking a few things though 

could I add a 12v 15ah battery lithium ion in series to get 60v battery output? Would i harm my existing battery ? 

I have seen cheap 12v 15000mah batteries on eBay designed for cctv systems or various things , could I add this to my bikes existing battery connection and turn it into a 60v like my controller is designed for? 


Also how far can you push a controller in terms of voltage, as I was thinking maybe I could get a 24v and go up to 72v . Would that be too much for a 60v controller? 

Thanks guys 

JAMES

 
I think the thing that is important in putting batteries in series is the ah rating. Make sure they are the same.

I'm happy for someone to correct me on this.

Regarding the controller. Personally I would not go above the recommended voltage.  60V in your case.  Unless you have money to spare and can afford another controller if/when it goes pop cause you have put 72V in it.

 
No. The lower rated batteries would run down first then try to charge in reverse polarity driven from the larger ones. In this sense they would be a resistor, as suggested and would detract from the power available, but it would also quickly ruin them and they could even explode or catch fire.. This can already happen with cells within a battery, but the maker should have ensured they are balanced, in terms of discharge rate/voltage. With different source batteries any close balance is unlikely.

Also do not believe the ampre hour ratings advertised especially with cheap batteries. Apart from being at least optimistic, (not to say lies), it is an application dependant rating. 

Batteries which can supply say 1 amp for 10 hours will probably not supply 10 amps for 1 hour.

Finally, never exceed the rated voltage of any semiconductor device or equipment. All too often the designers have left minimal safety margins, to keep manufacturing costs down.

 
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Wow thanks so much guys , I didn’t realise it would be that complex when combining 2 batteries together. I have seen guys adding batteries together to run their bikes at higher voltage. But based on your knowledge I’m definitely going to avoid it since it is so expensive. 

Does anyone have an idea how I can could achieve 60v battery from my 48v battery or would I need to just have a new one built to 60v spec ?  I just figured it must be possible to combine batteries. My current battery has a BMs inside of it to regulate the charging and balancing I think. Is there a way to achieve what I’m after in a safe way? 

Thanks 

james 

Also Does anyone know how to test a motors maximum input voltage and watts ?

thanks 

 
Very seldom would we just replace one faulty battery in a bank of batteries we would replace the whole bank, point being the batteries need to be fairly well matched for optimum safety and performance. I have seen the result of a battery going into reverse polarity many years ago and it wasn’t pretty. I actually had to ask one of the cleverer blokes to explain the failure as I didn’t understand it at the time.

Max Voltage of the motor should be stamped on the name plate, and barring failing bearings and or brushes if it is a universal motor, there is very little else that will deviate the motor from the name plate details. So max watts, current, rpm should be accurate.

 
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Thanks, so basically if I add a new 12v 15ah in series to the existing old 48v 15ah I would have issue with the older battery and the newer ones not being compatible due to their age and inability to keep up maybe ? And like Geoff said I would wear down the smaller battery first and then it goes downhill. Is this because I’m using lithium ion batteries with high discharge currents ? I guess there are just so many different batteries around it’s all amazingly confusing now. 

So so everyone is basically saying don’t try it and go for a 60v battery when it’s time to upgrade ?  No way of safely adding extra volts to the motor ? 

Thanks I really appreciate all the knowledge this forum is truly amazing 

 
Yes, you've summarised it OK. 

Whilst what you suggest with two unmatched batteries works in theory there are so many variables that there is a VERY high risk of it all going wrong.

I had assumed they were lithium ion types, hence my warning about fire, etc.  The same principle (of early failure}, would equally apply with other battery types.

The only batteries I would advocate using in series would be identical ones, of the same manufacturer and same age.

 
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