whats the Peukert like on NiMH, is it worth paralleling ?

[monster]

hi

i've ordered some serious amounts of NiHM and NiCd and i was wondering about how to connect them?

whats the Peukert effect like on these and is worth it paralleling with Schottky diodes?

i should have about 1kwh at 7ah and i was gonna have switches to turn packs on and off when they run out to stay below the 72v limit. but then i thought that if i drained equally from all i could switch then to series 220v for super fast charging without a transformer. if the Peukert effect is strong then that would be a double reason for doing this. with the switching option i'm discharging at 3C on peak power and my cells are only rated for 1.5C continuous?

 

[Ypedal]

alot of things to cover here..

1st.. without a transformer ?


2nd : It's best to paralell packs vs swapping out. keep the discharge below 2C unless you have some seriously good cells...

Don't ever discharge 100 % DOD... keep it no more than 80 % when possible.

Never let cells go below 1v per cell .. under load.. If they get too hot to hold after a ride.. you are pushing too hard.. if warm than it's ok, if cold you can push them harder ! ( Let them cool down before charging )

Charge in 36v banks if possible, and avoid paralell while charging. Have thermistors for the chargers vs " smart chargers " that guess the amount of cell you have.

Basic t hings to keep in mind for a happy Ni pack !

 

[monster]

thanks Ypedal i didn't know 2C but i've always suspected Peukert was affecting my range.

if i put all my batteries in series i will have 195v at full charge therefore i can charge with just a bridge rectifier and a light bulb straight from a 240v plug (scary i know!) i could charge at 1.5kw!!! or as fast as my bikes controller current limit!!!

here is my thoughts on parallel/series switching.

 

[Ypedal]

You plan to connect the pack to the wall outlet directly ?

I predict a big boom in your future !! :lol:

 

[monster]

nar it's cool. a light bulb will limit the charging current.

i charge at 72v at the moment -no problems. 88v max charge.

 

[Ypedal]

Hey.. if it works.. great.. please keep us informed of the project.. 8)

That said, i still think it's a better idea to use tried and true chargers. :?

It just sounds too simple to actually work.. Ni cells need to be trickled in order to ballance them, can't do that at high rate without spewing crystals..

Self discharge and lack of perfect cell ballance will be a problem i suspect.

 

[Ypedal]

Also, about the original question

The peukert effect with Ni is not very pronounced.. = Good !

Example : My 12ah SLA's will start to dip into the 10.5-11v per cell after 6-7ah if used with a 20amp controller.. gets even worse with a 40 amp controller.

My 8ah NiMh packs used to deliver 8.2ah when new.. after 2 years they still produce 7.5ah before the weakest cell drops below 1v under load.

The problem with my Nimh packs, they canot deliver 40 amps for more than a few seconds.. some better quality cells can do this.. but not the nexcell D 8ah cells i had on my norco.... 20 amps max.. 12 is more suitable.

Lead acid can dish out the amps for short periods..

Lithium is the best of both worlds ! (If you buy the good one's ! )

 

[monster]

the reason i love NiMH so much is because they are so simple to charge, you just pump at 0.5C until the temperature starts to rise and then slow down to 0.05C for an hour, then stop. i had standard chargers before, and this is all they did, but they were expensive slow and broke.

to make a good battery charger all you have to do is limit the current (and rectify) it's that simple and there are loads of ways of doing this, my favorite is the humble light bulb. a transformer is only needed for energy efficiency, it converts the mains voltage down to somewhere in the region of the battery pack so that you don't have to drop too many volts over a useless light bulb. if i lived in America where the mains is at 120v i wouldn't need a transformer because my 72v pack is pretty close to 90v at full charge and that means 75% efficiency. so then literally all you would need would be a bulb and a diode. i will soon have a 195v pack so i can do the same in the UK with our 240v mains, but my controller limit is 72v hence the switching.

NiMH are cool because only energy density and cost matters my controller limits power to 1500W, which is fast enough, so all the lithium in the world wont make me go faster. i only ever pull 20amp at 72v.

 

[Ypedal]

Well now.. you learn something new everyday.. 8)

 

[monster]

i've just read that its possible to wind your own transformers. sound like a lot of work, but it did get me thinking that it would be easy to unwind a few turns off a UK-US voltage transformer and make it step down to a lower voltage. i'll try this with my 100w transformer battery charger. here it is safely installed in a metal tin full of oil (for cooling) a nice portable charger. the tin is earthed, with 3amp fuse and every thing inside is insulated.

if i can make the secondary coil 92v it will make the charger more efficient, more powerful, charge-terminating on voltage and need only one light bulb.

 

[cerewa]

so then literally all you would need would be a bulb and a diode.

note: do not leave out the diode

I noticed that in Post#1, monster mentions charging off the wall outlets but doesn't mention using a diode. I'm not sure what would happen trying to charge a battery with A/C current, but I'm sure it wouldn't be pretty.

 

[monster]

i didn't give any details in the first post.