Charging warm batteries ?

[greasypants]

Just wondering how long I should let my 18 AH 36v NIMH packs cool down for after a hard ride before charging . After sitting for 2 hours they are still slightly warm to the touch so I have been waiting 3 hours before my charging timer turns on but then I can only charge them back up to 90% ( green light just comes on ) before I have to leave for work again . My chargers are the 36v 3A ones from ebikes , I probably should have gone with the 4a ones.

 

[knightmb]

Depends on how much spare time you have. If you let them cool down to room temperature they get the best charge. If you have to be somewhere in say 2 hours, might as well plug in the charger and get as much charging as possible. The only reason heat is a factor is that chargers use the heat sensor to determine when the batteries are charged (that and voltage). So if the battery is already warm, the charger will think the charging is done too early resulting in only a partial charge.

 

[Matt Gruber]

i'd use a fan.
as heat reduces the cycle life

 

[Lowell]

For 18Ah packs, the 4A chargers are definitely the better choice.

 

[Reid Welch]

Just aim a table fan at the pack to greatly speed its cooling so you can begin the charge a lot sooner.

This is a trick I learned in dealing with piano tuning---a cold or a warm concert grand delivered into a performance hall of different temperature simply cannot be tuned until the temperature of the 400 pound cast iron plate equalizes with the room. I learned to take along a fan, or even an electric leaf blower for rush jobs where the time available to tune the piano was short.

 

[TylerDurden]

Hurrah!

Two more good reasons to own a non-contact thermometer!

(In addition to checking motor temps.)





You can tune a piano, but....


8)

 

[greasypants]

Hey lowell did you notice any increase in capacity with your battery packs after the first few cycles ? Did a test ride to work and back last night and it looks like I'm using about 95% battery capacity on my 30K round trip. Don't have a drain brain yet but rode around the neighborhood after the ride and only got about 3km before the bike died .Only the 3rd cycle for the packs so far. If this doesn't improve I will probably add an 18AH 14.8V emoli pack or something similar.

 

[Ypedal]

I found that it's actually best NOT to drain the packs completely until you have at least 4 shallow cycles, leving the charger on trickle ( when light turns green.. it's still trickling ) for 2 more hours once green.. or overnight.

Forcing current into cells that are flat can cause cell reversal.

Picture in your head a row of water glasses... all full... left for a few weeks.. in a perfect world they evaporate at the same speed ( self discharge ) .. But.. lets say one side of the row is closer to a window, or exposed to sunlight.. it will evaporate faster , resulting in the water level of each glass being different.

When you charge for the first time you are charging at 3 amps until the fullest glass is full.. then the charger goes to trickle... the partially empty glasses will require a few cycles to top-off and be at the same level as the others.


If you drain your pack flat on the first few cycles.. you hit EMPTY on the glass that's in the window and the others still have water.. forcing the empty glass to be left out and hurt... Silly water analogy.. but you get the idea i hope. :lol:

 

[greasypants]

Thanks ypedal , good explanation. The first 2 cycles I only drained the packs about 50% ( based on km riden) and charged them overnight ( several hours with the green light on ). I did a test ride to work and back , did not plan on totally draining the packs ( no drainbrain yet), just rode around the neighbor hood once to make sure I had a 'buffer zone'. I was surprised when the packs ran out , I expected a bit more milage from them although I was riding at near 100% throttle most of the time . If a cell did go into reversal will I notice a difference in voltage between the 2 packs when fully charged ?

 

[Ypedal]

If a cell did go into reversal will I notice a difference in voltage between the 2 packs when fully charged ?

Yes.. but not right away, check the voltage before you charge.. mark them down... plug in to charge and make note of the time.. try to notice when the chargers shut down.. they should both go green within 10minutes of each other usually..

Test voltage again and mark down... next morning.. check again and if there is a 1 v + difference between them, something is wrong.. i doubt you have a problem after so few cycles.. they should be fine.. but treating your packs with care will pay off in the long run lol..

 

[Jay]

Just in case anyone's interested, I was at Harbor Freight today & they had a non-contact IR thermometer on sale for $6.99