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How can I measure the discharge rate on a 2000 NiMH pack? I only have a model 73 Fluke multimeter.

chuckk2

Most of the DVM's have a current measurement capability.
I don't know if the 73 has this function. If not you would end up using a low ohms value current "shunt",
and measuring the voltage drop across it.
E=I x R, I = E/R
E = Voltage drop across the shunt
I = Current
R = Shunt resistance

If you intend on really getting into Lipo powered models,
a power meter might be nice to have.

SkidMan

My Feedback: (17)

I use a West Mountain Radio CBA http://www.westmountainradio.com/pro...oducts_id=cba4

It will discharge batteries at a programmable rate and will graph the voltage over time displaying the total amp hours discharged. This way I can ensure that my batteries are up to snuff.

In fact, I just finished testing a homebrew A123 flight pack that unintentionally sat on the shelf fully charge for a year. I cycled it once and graphed it against its discharge curve from about 2 years ago and looks great with over 2,000 mAh under a 2.30 amp load and a 5.6 volt test cutoff voltage.

I understand the Powerlab chargers can do something similar.

chuckk2

The CBA is a bit pricey! $159-199
Battery monitor (already have one that will beep when low cell drops to programmed voltage)
Load one or two 12V 50W bulbs Parallel for 4C & below, series for 5C and above
Total cost of load (Bulbs, wire connectors etc.) $20 or less depending on bulbs used.

SkidMan

My Feedback: (17)

Yeah, the new ones are expensive. A year ago or so I bought the version 2 for $50 used and haven't looked back.

Like you say a battery monitor works fine for determining when a NiMH needs to be recharged. However, determining how much power it is actually able to discharge over time takes a little more work. Like Chuck points out you can measure the current by the voltage drop across a shunt and and by recording that and the voltage a timed intervals you can compute how many amp hours the battery put out before needing to be recharged.

I would, however, not try to discharge a NiMH battery at 4C and greater rates. When cycling a 2,000 mAH battery I would probably test it at 1/2A or maybe 1A.

Paul

chuckk2

A NMIH receiver battery should be tested at about the same discharge rate as it will have in use.
I'd think that a 2000mah NMIH would have a rated continuous discharge current.
The last time I looked into NMIH ENLOOPs the discharge rate vs. voltage drop limited the discharge rate to below 2A for a 2000mah battery.
High peak rates can cause digital receivers to reset, etc. due to momentary voltage drop.
For smaller NMIH batteries, my usual chargers, (BC6 and a four port) work nicely for charge slow discharge, and cycling.
Although, I generally only use NMIH and NICAD batteries for bench test. My A/C models are electric motor LIPO powered, so I use a combination of ESC BECs and external BECs as required.
For instance, electric retracts on a separate BEC.
The "higher" current light bulb loads are most useful when you have LIPOs that need to be discharged to storage.
Using a power meter in conjunction with a battery monitor that has an alarm will easily tell me what amount of charge (mah) was available between full and storage levels.

SkidMan

My Feedback: (17)

Thumbs up!

Based upon the 2000mAH size I'm guessing that these are AA sized cells. I hope this means that it is going into a relatively small aircraft with relatively low power requirements. If the plane is larger, has a lot of powerful servos, is gasoline with an electronic ignition, or is flown in a manner that requires a lot of power (e.g. 3D) I would recommend a different battery chemistry. Personally I would go A123 or LiFe but even NiCd's can handle higher current loads better than NiMh. While not all NiMh cells are created equal, I am comfortable using NiMh in planes with 4-5 analog standard servos (hence the 0.5 to 1.0 amp discharge suggestion - the actual flight current may peak above and below those values.).

Like you wrote in this post, the 50watt light bulbs will work well for Lipos. However, I would not advise using them with 4-5 cell AA NiMH packs. Lipos are capable of much higher discharge rates than NiMh. Sustained 4-5C discharge rates are nothing for most lipos but are above the call of duty for most AA-sized NiMH cells.

WestCoastD

Can I use my older (popular) ACE R/C "DigiPace" nicad cycler to cycle LiPO packs?

I am primarily an RC boater. I have used several SR brand nicad packs-- 500mah , 600mah, 1000mah 6V Rx packs, and 600mah 9V Tx packs.

Will the LiPO pack characteristics/specs prevent proper voltage cut-off or charge rate? Hope I'm expressing myself clearly here.

shonny

How about using the ESC and some light bulbs of appropriate voltage and wattage plus a stop watch? That way at least you don't risk discharging the Li-Po too much. And you know when the batteries are discharged in that the light bulbs are switched off.

Warjet

My Feedback: (327)

Unless you want to see fireworks and start a roaring blaze, you never charge anything lithium on a Nicad / Nimh only charger. The charging process is completely different. Just get a $40 LiPo / LiFe / Li-ion charger that will also do nickel packs and you're in business.