charging info for 36v battery

[barrycoll]

as an e-idiot when it comes to understanding the wonders of electricity, could some MSc or PhD out there explain why a 36v battery shows 40-41volts on the Multi tester after charging , and 39 volts after a 30 mile run....what would it show if it was really run down??

I have only ever charged the battery for 2 hours after a run, to keep it half/three quarters full between uses (of maybe 2-3 weeks apart)....the battery only has 2 green lights on it, and they have never gone out, so I dont get much info from that direction

to keep it at optimum between uses, what sort of figure should the Multi Tester show for storage?

thanks barryc.

 

[banbury frank]

Hi

your battery pack is made up off 9 cells or banks of sells each charged to 4.7 volts
they will then drop to 4.2 volts

when discharging The lights are just an indication

Frank

 

[NRG]

They types of lithium cells used in our batteries have a 3.6 or 3.7v nominal voltage so your 36v battery will be made up of 10 cells @ 3.6v (not sure where Frank got 9 cells from). At full charge the pack should read approx 42v.

I don't see the point in only charging to 75%, you have limited capacity as it is so theres little point in limiting it further....also you may be unintentionally causing the cells to become unbalanced by not allowing the charge process to complete. Many batteries (the BMS) will balance the cells in the last stages of the charge process so by cutting the charge cycle short you will be cutting out this vital last stage of charging.

2-3weeks between runs is no problem, you only need to limit the capacity to around 40% if planning on long term storage IE Many months.

The low voltage cut off will be set around 31v or so to protect the cells, at 39v you still have plenty of capacity left...

as an e-idiot when it comes to understanding the wonders of electricity, could some MSc or PhD out there explain why a 36v battery shows 40-41volts on the Multi tester after charging , and 39 volts after a 30 mile run....what would it show if it was really run down??

I have only ever charged the battery for 2 hours after a run, to keep it half/three quarters full between uses (of maybe 2-3 weeks apart)....the battery only has 2 green lights on it, and they have never gone out, so I dont get much info from that direction

to keep it at optimum between uses, what sort of figure should the Multi Tester show for storage?

thanks barryc.

 

[flecc]

The majority of nominlly 36/37 volt lithum batteries use ten 3.7 volt cells in series, making the nominal 37 volts.

As you've noted, off the charger they can measure about 41 volts and still 39 volts after some usage. The control systems on the e-bike cut the current when the voltage drops to below 31.5 to 32 volts, in order to protect the cells from over-discharge damage.

All rechargeable batteries record higher than nominal volts when first charged. NiMh cells which are nominally 1.2 volts are at around 1.5 volts when charged, 12 volt car batteries record well over 13 volts when fully charged.

N.B. crossed with NRG's post.

 

[parker]

Hi

your battery pack is made up off 9 cells or banks of sells each charged to 4.7 volts
they will then drop to 4.2 volts

when discharging The lights are just an indication

Frank

Hm... very interesting! I didn't know that!

 

[Scottyf]

In the BMS though the cells are kept in balance all the way through the charge. That was my understanding.
As they are always attached to the BMS the cells should be increasing in volts while still being balanced.

 

[NRG]

Depends on the balance method, if passive IE resistor bleeding it's done near the end of the charge sequence, charge pump or cap switching in theory can be started earlier in the charge cycle but given the large CC used prior to switching to CV it would not be able to keep the cells in balanced AFAIK. If anybody knows different then I'm all ears!

 

[barrycoll]

Mnay thanks to Dr NRG and Prof Flecc, as the basic explanation finally has made sense to me..

its a pity you had to spoil it all with talk of charge pumps and cap switching.!

as another rather simple aside, the 37v battery that came with the Electric Wheel Nano Brompton, is obviously a 'behind the seat tube' jobbie, but fits the Brompton bag perfectly

the exposed electrodes (on the base of the battery) that would normally contact the male part on the bike, must have been disconnected, as there is now an insert-and -twist connector at the other end of the battery, which detaches from the lead mounted in the Brompton bike bag, but I dont seem to be able to find a live outlet when poking around with the probes from the Multi tester, so no reading.

Any ideas folks, about why this is so???

Thanks again for all your time...........barryc.

 

[flecc]

Have a closer look at that connector, you might find a tab, button or slot with corresponding ridge that disconnects the supply when uplugged, for safety reasons.

 

[banbury frank]

Hi NRG

Just out off interest It depends on the type off cells used we now use Lithium polymer battery's and our 48 volt battery's run at 54.6 volts /52 and 13 X 10AH cells Each cell 4.2 volts This helps to keep the weight and size down

so if we supplied 36 volt battery's 9 x 4.2 volts 37.8 volts we no longer do 36 volts as it is all about

Watt hours for size so 52 X 10 = 520 WH for 5 kilos in weight

Frank

 

[flecc]

Yes, there is that different kind of lithium cell that has a nominal 4.2 volts Frank, but hardly anyone on e-biking uses them. The almost universal standard in Li-manganese/polymer is to use 7 x 3.7 volt cells for 26 volts to power what were 24 volt systems and 10 x 3.7 volt cells for the old 36 volt systems, some manufacturers like eZee and Wisper now referring to them as 37 volt.

In some cases two groups of series connected half capacity cells are used n parallel, 14 (2 x 7) x 3.7 volt 5 Ah cells in a Panasonic 26 volt 10 Ah battery and 20 (2 x 10) x 3.7 volt 7 Ah cells in the eZee and Wisper 37 volt 14 Ah battery.
.

 

[barrycoll]

thanks again for that further advice flecc...

after studying the male plug, I managed to find the terminals in the socket, and the 2009 battery is still showing 41v....

so now I suppose I should finally follow NRG's advice and top up both batteries to allow the BMS to do their stuff....

you are a great help.............many thanks barryc.