Cycling NiMh- discharge methods?

[BHChieftain]

Anybody have a nifty way to discharge a pack without actually running it down with an engine? My charger does not have a discharge function, and I'm never sure if my battery gets completely discharged when running it down with my ESC/motor (plus when I'm at home, it sure is a hassle holding the boat down in the bathtub--and the motor will overheat without cooling water)

I've heard of folks wiring up the battery to a light bulb to act as a resistor, but I'm not sure what wattage light to use...

-Chief

[m4a1usr]

Nimh packs do not have issues with memory as did Nicads where charging/ discharging them was an important maintenance issue. Why do you feel the need to discharge them? Charge them once a month when not being used and they should be good for a long time providing you dont get them too hot during use.

If you want to cycle them, look for an AstroFlight 112d charger. Simple by todays charger standards but built like a tank and will do exactly what you want to do.

John

[BHChieftain]

Hi,
The reason I want to discharge them is one of my 2 new packs won't accept a fast charge-- it stoped after 15min. I thought that pack must have been partially charged, but when I put the two packs in my Fastech, the ESC low volt cutoff kicked in after 1 minute. So that battery must not have charged correctly.

So, to ensure a full charge, I need to completely discharge the battery and recharge using a trickle, and measure the mah to make sure I have 2 balanced batteries.

-Chief

[egneg]

I made one using 4 #1157 auto bulbs soldered to a couple of pieces of solid 12 awg wire. I will post pics tomorrow if needed.

[bwells]

I made a discharger with a 12 volt fan that you see in the windows of the motorhome guys. Real cheap and a parallel wye, I discharge 2 8.4 NiNM in 30 minutes. Can't remember why I do this though!!

[BakedMopar]

Hi,
but when I put the two packs in my Fastech, the ESC low volt cutoff kicked in after 1 minute. So that battery must not have charged correctly.

-Chief

If your ESC is set to Lipo with LVC it will always cutout when running NiMh. When running NiMh you should dissable you LVC.

[Jimmy Sterling]

NiMh Storage

NiMh cells are not to be stored fully charged, doing so causes the cell to develop what is known as dendrites which is the main cause of internal resistance. This internal resistence is what eventually leads to the cell discharging itself over time.

The most proficient way of avoiding this is the store the cells @ a recommended nominal voltage of .90 volts. When I was using NiMh cells I used what is known as a discharge tray by Novak. A 6 cell pack could be placed on the tray and discharged to .90v and retained @ this voltage, after every race meeting all of my packs would be placed on the tray and discharged over a period of 4-6 hours.

The Novak discharge tray was a very worthy investment @ the time and I am sure you could pick one up on eBay fairly cheap as many people have moved on with the Lithium technology,

IMO, the best way to ensure a long life out of your cells

Jimmy

[BakedMopar]

Novak discharge try will only work with side by side packs if your using shotgun style packs just hook it up to some auto bulbs like suggested. They are easy to make and fairly inexspensive. The only down side is if you leVe them unattended they will overdischarge your cells.

[BHChieftain]

If your ESC is set to Lipo with LVC it will always cutout when running NiMh. When running NiMh you should dissable you LVC.

I'm running a stock Proboat Fastech, the default settings are for NiMh (I do have a programming card for it but have not used it yet). For the NiMh setting, there is a low voltage cutoff which enables either a .7V or .9V Nimh cutoff, which enables you to have a "reserve" to nurse the boat back to shore when the batteries run low. When in Lipo mode, the LVC range is 2.7 or 3V cutoff.

Pretty nifty feature for nimh, as I've had to fish for my other boat more than a few times...

-Chief

[BHChieftain]

I made one using 4 #1157 auto bulbs soldered to a couple of pieces of solid 12 awg wire. I will post pics tomorrow if needed.

That would be useful,

-Chief

[BHChieftain]

NiMh cells are not to be stored fully charged, doing so causes the cell to develop what is known as dendrites which is the main cause of internal resistance. This internal resistence is what eventually leads to the cell discharging itself over time.

The most proficient way of avoiding this is the store the cells @ a recommended nominal voltage of .90 volts. When I was using NiMh cells I used what is known as a discharge tray by Novak. A 6 cell pack could be placed on the tray and discharged to .90v and retained @ this voltage, after every race meeting all of my packs would be placed on the tray and discharged over a period of 4-6 hours.

The Novak discharge tray was a very worthy investment @ the time and I am sure you could pick one up on eBay fairly cheap as many people have moved on with the Lithium technology,

IMO, the best way to ensure a long life out of your cells

Jimmy

Do you need to strip away the packaging material for your nimh packs to use it? What do you use to put 'em back together?

-Chief

[egneg]

I had a hard time finding this as it has been quite some time since I have used it. If you want it I will send it for the price of a small flat rate box USPS. But then again it's not hard to make and the bulbs might not make it in the mail.

[BHChieftain]

Thanks for sharing the design! I think I can build one of these...
-Chief

[sundog]

Here's one I made to discharge Lipos that don't get used that day. I have a B6 charger and my only gripe is that it discharges so slooooowly. This will take a large (5000mah 30C) 2s or 3s to storage voltage in short order. I just plug a lipo saver (low voltage alarm) into the balance plug and unplug it when it sounds off. It is made from 10) 1157 taillight bulbs and uses both elements in each bulb - about a 25A load total. It got hot enough to melt the soldered connections so I installed a CPU fan to keep the heat down. Was cheap - I paid about $5 for a box of bulbs (from ebay) and maybe another $5 in the deans, fan and wire. Works well.

[785boats]

I had a Johnson 785 motor with a 8'' X 6" pitch aeroplane prop on it mounted to a bracket that I held in the vice to discharge packs when needed. It would draw 12.5A on 6cells & 24.5A on 12 cells, when fully charged
I would set it up with an ESC & radio & check the voltage as the pack ran down on full throttle.
When the voltage dropped to 0.9V /cell (5.4V for 6 cells or 10.8V for 12 cells) I would pull the throttle to half.
When the voltage dropped to 0.9V/cell again I would pull the throttle to 1/4.
Then to 1/8 throttle until it hit 0.9v/cell again.
Then I would shut it down. I considered the pack to be discharged.
In the last few stages the packs were being discharged under very light loads which helps balance the cells.
But since I run Lipos in all my boats, including my fleet of scale boats too, I don't need to bother with that anymore. Got rid of all my metal cells.
But it use to work for me.
It would work for Lipos too for discharging to storage voltage, but my charger will do that for me at a 5A load automatically.
Cheers.
Paul.