Converting 48v to 36v

[my.motion]

Hi Team,

I have a spare 48v 12Ah Lithium Ion battery pack which I wish to convert to 36v. The pack consist of 39 cells. Will this be possible?

http://www.pedelecs.co.uk/forum/electric-bicycles/4588-battery-cell-issues-pls-help.html#post58340


Will i require a new BMS?

Thanks.

 

[Straylight]

I'm no electronics expert, but my first thought would be to see if there's a dc to dc converter to achieve this rather than hacking your existing pack about. I'd have thought this would have a side benefit of placing less strain on the pack and extending it's life, but this is pure conjecture.

 

[NRG]

I think you will struggle to find anything off the shelf with the required current rating in a small package to fit your needs. You may have to go DIY or get something built to handle the current....then it still would be a struggle to keep the size down due to the possible heat sink requirements.

The last cct here looks interesting though and certainly capable of handling the current with the parts listed...

High Current Voltage Regulation - Electric Circuit

 

[flecc]

There are three parallel groups each of 13 cells in your battery, and 3 cells need to come off each group to leave it as three parallel groups of 10 cells to make 36 volts.

However it's likely that the present BMS will see the missing cells from it's balancing circuits as an error, but whether that will stop it from balancing the charge conditions of the remainder I can't say, not without knowing much more about that BMS.

A suitable 36 volt BMS would be better, but finding a suitable one could be problematic.
.

 

[emissions-free]

As fless said. It would be pretty straight forward to remove some series cells to make it 10 series cells instead of the likely 13 or 14 in series you now have but the BMS probably wouldn't work. If you can find someone reasoably knowledgeable about cell configuration they should be able to determine the suitability of the BMS for 36 and reconfigure the pack for you. You should actually be able to increase the Ah rating as the series cells that are removed could be added in parrallel.

For example I have some LiFePO4 packs that contain 24 cells, each cell is 3.2V and 3Ah. Some of the packs are configured 8S (series) and 3P(parallel). That makes a 24V nominal (actually 25.6V) with 9Ah. Also have the same 24 cells configured in 12S, 2P. This gives you 36V nominal (actually 38.4V) and 6Ah, but a different BMS is required. However a 36V BMS with say a 20A rated current is not so expensive

If you are fairly handy with a soldering iron and fancy a bit of DIY, it should be possible to do it yourself. I could try to give you a hand sourcing a BMS if you're not in a hurry.

Why do you want to convert to 48V anyway? A different controller may give you what you require and be a lot less hassle.

 

[my.motion]

As fless said. It would be pretty straight

Why do you want to convert to 48V anyway? A different controller may give you what you require and be a lot less hassle.

I am looking to use this as a backup on my Milan2 bike that has SLA Battery. I have been charging the SLA battery twice a day for the last 4 months, hence looking to get ready for a replacement. What is the max current I can pass through the Milan2 controller without damaging the bike? I think the Li-ion has 12Ah

Thanks

 

[emissions-free]

The Ah rating of the battery doesn't directly determine how much current will be delivered to the controller. The voltage is the more important factor. I try to explain.

If your current controller and motor combination are pulling lets say 10A at 36V, that will equate to 360W being pulled from your battery. That is likely approximately the peak or max power that is delivered for a regular 250W motor. A typical 36V 250W rated will have a current limit in th region of about 10 to 15A.

If you increase the supply voltage to say 48V, that will cause more current to flow. Power = volts*current. If you increase the voltage by 48/36 or 1.33, you will also increase your current by the same amount so it will now deliver 13.3A at the higher voltage rather than 10A at 36V. This results in a power increase from 360W to 360*1.33^2 = 640W. So the power is now 178% of the original standard figure.

The limiting factor is possibly your controller but most can accept a step up from 36 to 48V. It will also put more strain on the motor and the type of motor used will effect how suitable it is fo overvoltage. All the above is asuming you are using a typical hub motor.

There's a bit more to it than the above, but don't mind giving you some pointers if you want

 

[my.motion]

Thanks. The other problem I have with the battery pack is that 2 or 3 cells seem to discharge quicker than the rest. This pack had problems with BMS and I have now a replacement pack, and got to keep this faulty pack. I have seen an Infenium controller that is congigured for 29v-48v that I can use but it was quite expensive. How much is the 36v BMS please?

 

[emissions-free]

I'll look into it a bit and see if there's a reliable source in the UK. I'm ordering some BMS as samples, but I'm in China. They're not that pricey.

I wouldn't attempt it unless you're fairly handy with electronics though. The cells are generally connected together by metallic strips which are spot welded onto the cells. I reckon you could just cut the strip, leaving something that you could solder onto or even put a hole in it, put a nut a bolt, then cable with appropriate crimp and cables. You shouldn't solder directly onto the cells or get them too hot. A big soldering iron would be required so it could heat the area very quickly without letting significant heat get to the cell.

Whatever you do, make sure you take care. There's alot of potential energy in 1 of th packs, so don't wera jewelry and try not to drop a spanner on it

There's lots of very knowledgeable people on Endless Sphere. They could point you in the right direction if you get on there and have a look.

Best of luck

 

[my.motion]

Emissions-Free/All, Thanks for all your help, much appriciated!!