volts, volts, nonimals, charges? WTH?

[emaayan]

ok, every time i got this, i don't.

there was a time i used to think batteries were really 48,36,24 volts, but now i understand they aren't,and these are nominal voltages., that the real max voltage is what considered charge voltage , and 48 for example is some where in the middle between charge and cut-off voltage. but then came along the the concept of 52 volts batteries which in fact i was told is 48 volts, so what's the difference?

then, i was also told the lifepo batteries should have a charge voltage of 58.4 , but now i see, that i could be in fact either 56.7/58.4/59.2, (16 serial cells) all depending on the charge volt of the cell (3.6,3.65,3.7 respectively) but it's hard to know from specs of the cells which are EXACTLY that voltage).

not to mention the fact that when i charge my current (48v) battery , the voltage meter shows it can reach 58.5, but when the charger stops charging, it gradually drops to 53.1. why?

 

[Cold-E-Commuter]

The nominal (selling/advertised) voltage of packs seems to be on the antiquated 3 volt per cell system. This does not necessarily mean that is the actual voltage of the pack. LiFePo4 for example are considered widely as 3.3 volts per cell, so a 16s pack would be 52.8 volts. For sales purposes, 48 volt. As far as the charge, it depends on the charger and battery chemistry. I have 2 LiFePo4 chargers. One cuts of at 3.6 volts per cell and the other at 3.7. Not a clue as to why. Maybe others will. Good post for a scattered answer.

 

[wesnewell]

Here's a good place to start.
http://en.wikipedia.org/wiki/Rechargeable_battery

 

[dnmun]

the controllers were developed based on using 12V SLA batteries. so they go in 12, 24, 36,48V increments. for lifepo4 everyone uses 3V as the nominal voltage and a 16S lifepo4 pack is considered a 48V pack but actually is 58.4V when fully charged to 3.65V/cell.

the chinese ebay sellers have now started using the term 48V for a 10S limn2o4 pack which charges to 42V. and they call everything lithium ion to make sure nobody catches on.

 

[emaayan]

so why do my battery drops gradually to 53.1 after the charger stops charging it?

 

[dnmun]

that is hard to answer without knowing how you pack is balanced. to answer specifically you need to measure the voltage on each cell while it is charging. it may be outa balance and you are taking it off the charger when the charging light turns off before it can balance. no way to know without the cell voltages while charging

 

[dogman dan]

I think you got the nominal thing right. And you know the charged voltage is what the charger is actually putting out, combined with when it stops. A charger stopping early for example, might result in an immediate drop in pack voltage as soon as it's removed. The charger might be putting out 58v, but if it stops too early the pack never got there.

Many will say lifepo4 charges to 3.65v, and depending on charger and bms settings it should, or perhaps even slightly higher.

But as you saw, it does not stay there. That is because a slight overcharge makes the battery balance better with the bms system in use. The excess charge is called surface charge, which you can now look up and research if you want to. So your battery is not expected to rest at 58v for very long. It should drop to 56v or less the instant you touch the throttle, or drop several volts over the next few hours.

Assuming lifepo4, 53v divided by 16 is 3.3v so that is lower than just surface charge dissipating should be IMO. 54-55v is more like normal good cells fully charged after surface charge is removed.

Why this is, assuming your pack is fairly new needs to be investigated as said above. One possibility is the charger turns off early, another that you aren't leaving it on the charger long enough to balance, like overnight every night for weeks in some cases, or perhaps you actually have a weak cell that won't fully charge, or several, that is dragging your pack voltage down 2v.

 

[emaayan]

FalconEv says it's normal , the charger i got doesn't seem to turn off early (unless off course you mean it shouldn't stop at 58.5 but rather at 60).

 

[dnmun]

not what i meant. if some of the cells are all around 3.9V when the charger shuts off then the BMS will drain them down to 3.6V right away. or the pack could be balanced and the entire pack drops just from the movement of the free lithium ions in the electrolyte back to the anode where they are deposited. what people call 'surface charge'.