what is 40% charge??

[barrycoll]

a question for The Wise Ones

40% of a full battery charge is suggested as the possible level to leave a battery that wont be used for a few months...

my wife is off to Oz for a time to see family, and her Nano Brompton 36v battery is showing 39.9 volts on the digital multi meter.......and shows as one light, on the 3 light display on the battery

now 41-42v seems to be a full charge, and 36 is the nominal generic amount, so what would a reasonable level of charge be for best storage...

........and how does one work out a 40% level????

many thanks.................barryc.

 

[flecc]

Unless you've previously measured the voltage by stages during a charge period Barry, a voltage measure won't be very accurate. However, since the recommendation for storage of lithium batteries is between 20 and 40% of charge, precision isn't needed and it's probably in that zone at present with one LED lit.

Please note though that the battery can only safely be left for three or more months if refrigerated at very close to zero degrees C. If stored at room temperature for more than three months it's quite likely to be useless on return, since the residual current drain of the battery's internal BMS can take the cells well below safe level.

Can you arrange for someone to charge it for at least one hour once every two months to keep it working?

 

[barrycoll]

thanks a lot flecc

would the battery be better off, if charged up to 2 LEDs and then refridgerated??

.....or leave it at one LED, refirdgerated

.....or given one hour on charge, at room temp, every 2 months????



when you say measure the charge progressively while charging, I take it you mean every hour (say) from a fully depleted starting state????

thanks again for your time

cheers barryc.

 

[flecc]

For safety I'd say charge it until the second LED just lights which will be as accurate as any other method of getting to around half charge, then recharge every two months to the same two LED just lit level.

If storing at low fridge temperature as well, allow it to restore to room temperature for a few hours before the two monthly charges. Low temperature storage is mainly to prevent loss of capacity rather than loss of charge.

That interval measuring during charge was what I meant, but it's still inaccurate since tbe rate of charge declines as the cells fill. Therefore I'd just use the second LED just lit method.

 

[tillson]

........and how does one work out a 40% level????

You could weigh the battery when fully charged and then weigh it again when empty. Subtract the empty weight from the full weight, multiply the result by 0.4 and then add that figure to the empty weight.

This will give you the weight of the battery when it is 40% full of electricity.

C40 = ((Cf-Ce)*0.4)+Ce

Where, Cf = the weight of the battery when fully charged, Ce = weight of battery when fully discharged and C40 = the weight of the battery at 40% charge.

I'll get my coat and leave.....

 

[indalo]

You could weigh the battery when fully charged and then weigh it again when empty. Subtract the empty weight from the full weight, multiply the result by 0.4 and then add that figure to the empty weight.

Obvious really Tillson! I don't understand how the OP couldn't work that one out.

Indalo

 

[flecc]

I'll get my coat and leave.....

. Unfortunately it still wouldn't be dead on, since "empty" is only at the level the battery allows itself to be discharged to. That quantity of residual charge then is anybody's guess.

Whatever we do it will always be an approximation.

 

[bode]

Whatever we do it will always be an approximation.

In life generally, and not just in relation to batteries...

 

[JuicyBike]

How about water displacement?
I'd go with Flecc's "2 lights showing" rule Barry.

 

[tonyishuk]

Maybe a silly question, but is there any reason why the battery >> charger could not be connected to a seven day time clock and charged for a two or three hour period in a week?

Rgds (from a Newbi)

 

[flecc]

Maybe a silly question, but is there any reason why the battery >> charger could not be connected to a seven day time clock and charged for a two or three hour period in a week?

Rgds (from a Newbi)

In principle no, but the charge would climb to full in no time at that sort of rate. Ideally it would be a monthly charge of no more than an hour, but I think most timers are weekly. Probably 15 minutes a week would be enough to prevent low voltage cell death in storage.

However, lithium batteries are not unconditionably stable and charging while completely absent is not a wise thing to do since they can under rare circumstances be a serious fire risk.

 

[Synthman]

Maybe a silly question, but is there any reason why the battery >> charger could not be connected to a seven day time clock and charged for a two or three hour period in a week?

Rgds (from a Newbi)

Yeah I thought of this too. Maybe a more advanced digital timer that you could specify the exact dates to switch the charger on would work (if such a thing exists). With the normal timers maybe just move one or two segments on the dial so it comes on 5-10 minutes a day.

 

[bode]

Maybe a silly question, but is there any reason why the battery >> charger could not be connected to a seven day time clock and charged for a two or three hour period in a week?

I do just this with my lesser used battery, but for half an hour per week just to keep it fully charged for when I do want to use it.

 

[tonyishuk]

However, lithium batteries are not unconditionably stable and charging while completely absent is not a wise thing to do since they can under rare circumstances be a serious fire risk.

Thanks, Food for thought whilst letting the batteries recharge at night whilst we are asleep in the motorhome ! ;-)

Slightly O/T but some battery chargers for m/homes have a temperature pobe to monitor battery temperature, sounds as if these bike chargers could do with similar thinking !

Rgds

 

[flecc]

Slightly O/T but some battery chargers for m/homes have a temperature pobe to monitor battery temperature, sounds as if these bike chargers could do with similar thinking !

NiMh e-bike battery charge cutoff is commonly by temperature, this indicated by an NTC thermistor in the battery pack which passes a cutoff signal to the charger when charge complete. This is because the rise in voltage at the end of charge is too gradual to give a repeatably correct cutoff point.

There's no equivalent for lithium batteries, the control there being by the battery's own electronics (BMS) monitoring the charge level. Any fire risk fault when it occurs is chemical rather than electrical.