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Mad Ax

Best way to charge NiMH (for long life)?

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TL;DR

What's the best way to care for NiMH when it comes to charging, use, and storage?  This is for 1:14 trucking, so I don't need high-current demands, I just want a set of packs that will take as many cycles as possible before going the way of the recycling centre.  I don't need to fast charge - I'm happy to slow charge before events, two packs per truck will last a whole day.

Long Version

It's been a while since I seriously used NiMH batteries in anything - since moving to LiPo I haven't looked back.  But I've got a big truck event this weekend, and due to an administrative error some new LiPos didn't arrive on time, so I bought two new Ripmax 3000mAh NiMH packs from the LHS.  They cost me a fair bit - as much as the non-delivered LiPos, which have almost a 40% higher capacity and are, well, LiPos.

Back in the good old days, my charging technique for NiMHs was "run it until it slows down, then crank up the clockwork dial to 15 mins and let it run."  Remember those clockwork chargers?  Terrible, weren't they?

When I was racing (and even bashing) on NiMHs, the general attitude was "do a race, then stick it on the fast charger, it'll shut off when it's done."  I ran for a few years like that, but mostly I had around 10 packs to choose from and at any one time, a couple of those would be suspect - i.e. not holding a charge or dumping very quickly when used.  While some NiMH packs would give me several years of use before heading for the bin (or light duties powering LED displays on the shelf), others would become troublesome within a few months, especially if they were being raced hard and charged hard.

The label on the battery says to charge for 14-16 hours at 0.3A (10%).  Yesterday I charged both brand new and unused packs at 0.3A and one had a false peak after 70 mins, the other charged for over 7 hours before peaking.  This morning I topped off the early-peaked pack at 1A and it took another 60 mins or so of charge.  Slightly worrying that these are both brand new packs yet exhibit different characteristics on the same dual charger.

So, what's the best way to keep them living long?

Should I trickle-charge them at 10% and let the better cells thermal off any excess?  I've heard NiMH cells can be damaged even on a trickle charge.

Would I be better charging at 0.1A if I'm going to trickle them?  That's a very long charge time :o 

Should I charge them at 1 - 2A and let the delta peak stop the charge?  Or even fast-charge at 3A?  What's the best delta to use to protect the battery (vs getting max voltage when the start buzzer sounds?)

Should I discharge them before recharging?

Should I build a discharging board to discharge each cell individually, or hook the battery up to a low-current application (like running the truck's lights on the workbench for a few hours) to zero all the cells before charging?

Is there any mileage in charging each cell individually with a probe, or is that only worth doing if the pack starts to exhibit poor charging / capacity characteristics later down the line?

I know there are people here who religiously use NiMH all the time and don't complain about the kind of problems I used to have 10 years ago, so there must be a way to treat them well, especially if I'm not fast-charging them four times a week between races and topping them off until they're boiling to get the very best performance off the starting box.

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This probably isn't very helpful, but I have 2 NiMH packs which have been abused for 2 years and show no signs of slowing down. Initially I only had basic triclkle chargers, the kind that plug into the wall socket and to the battery with nothing in between. They are 900ma chargers I think, and I just leave them on for hours until the batteries are a bit warm. I check voltage with a multimeter to make sure its close to topped off. I now have a couple of decent chargers and charge them like my lipos - at 1C - and haven't had any problems.

I also just run them til one won't move a car, then put both on charge.

So in short, these things get properly used and abused and are still going strong after what must be 200 cycles each. They were well under half the price of my lipos too!

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Generally speaking storage of NiMH is similar to lipo, anything around 25-50% charge is ok. 

For charging NIMH is more sensitive to overcharging than Nicad. But not as sensitive as Lipo. 

A peak charger basically detects when the voltage of the battery begins to fall, the reason the battery voltage falls when the battery is charged is because at full capacity the battery starts to inverse it’s polarity and resistance in the pack starts to climb as it resists this process lowering the voltage vs the input current. 

Nimh was less capable of handling being overcharged but also reached charge sooner and more predictably. The biggest difference between a NiMH peak charge setting and a Nicad peak setting was how long the charge would put up with the voltage fall before stopping the charge. Ultimately the physical numerical figure of allowable voltage drop is less on a NiMH.

Back in the day we never really charged our batterys slow, generally 1x capacity was very safe, but racers wanting a bit more punch might charge at 2x capacity. I see minimal advantage in charging very slow, apart from its much harder to over charge it. 

Cycling a battery can sometimes recover lost capacity, for racing we used to discharge completely before recharging, if you don’t then the recent charge is punchy but the residual charge capacity felt flat. Cycling the pack helped keep the power delivery more uniform across the entire capacity of the battery.

Cheers

Juls

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I generally charge my batteries at 1C I've been running NiHM for many years. I used to just slow charge them all until I got a new fancy charging thing. Charge at 1C. Run until the car slows. Then either discharge fully followed by a recharge or just charge again. I keep my batteries fully charged in storage as its just easier to grab one and throw it in a car for the kids to use. They don't quite understand the having to wait for batteries to charge just yet. Despite my best explaining.

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Thanks for the advice peeps :)

So I had a big event yesterday, hence buying two new packs.  I'd expected to get through a few charges, running two rigs all day with full lights, including leaving them switched on while parked (with the MFU in engine off mode), mostly because the layout is so packed and hard to access once the show starts that I can't get out to turn the trucks on/off.  Well, as it happened, it was so crazy busy that I didn't even start the rigs up until gone 1pm, and only went around the layout a couple of times before parking them up.  So I arrived home last night with 2 completely full packs and 2 almost-full packs.  Looks like my rigs will get some lights-on time on the shelves over the next few evenings :o 

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