Nimh load tester

[Brian Dorricott 1]

I've noticed that not many fliers have the old Nimh tester that puts a load on and displays on leds - good or bad. I rummaged around and used a cheap voltage display ( 3-30+ volts) and a switch that puts a 20ohm load across the battery . This gives a current of about 300mA on 6v and 250mAh on 4.8v a good load .

What would you think an acceptable voltage drop at that current draw should be ? I was thinking no more than 0.2 to 0.3 volt drop on a fully charged battery , anyone else got any thoughts ? 

[Cuban8]

Not all Nimh Rx batteries are equal. Some high capacity Nimh types will have a surprisingly low C rating and will have trouble sourcing little more than an Amp without excessive voltage drop. Others are much better, but as manufacturers seem rather coy about giving C ratings it's often difficult to draw meaningful conclusions.

As long as my nimhs can maintain 3A at the nominal 1.2V per cell for at least a minute from full charge, I'm happy. Regular sports type models will draw very much less  than that with normal flying and with non-specialist analogue or small digital servos. A longer full discharge test to 1V per cell at say 500mA will be informative if discharge times are noted and any major difference developing over time will be a warning.

A simple load test will only give half of the story - individual cells within a pack need to monitored for early voltage drop which is a sure sign of a weak and failing cell. Not so problematic with a five cell pack that can 'afford' to lose a cell and still carry on .........often unnoticed until a second cell goes the same way in an ancient and well used pack.

Losing a cell in a four cell pack that doesn't have that redundancy' can be disasterous.

Hope this of some use.

 

[J D 8]

I have used for many years an SM models load checker as described above and would say the thing to take notice of is a change in battery behaviour  when put on test like not being fully charged when it should be or faster than usual drop off when loaded.

 Any batt that changes is out.

[Nigel R]

15 hours ago, Brian Dorricott 1 said:

What would you think an acceptable voltage drop at that current draw should be ? I was thinking no more than 0.2 to 0.3 volt drop on a fully charged battery , anyone else got any thoughts ?

 

Every so often, you could use a multi charger to run a charge/discharge/charge cycle test to see what state the pack is in.

[John Lee]

The attached Data Sheet for Enerloop batteries shows the expected voltages at 400mA, 2A & 4A discharges. It should give you a good baseline and you can try searching for similar data for the cells you want to measure.

 

Enerloop data.pdf

[Cuban8]

1 hour ago, Nigel R said:

 

Every so often, you could use a multi charger to run a charge/discharge/charge cycle test to see what state the pack is in.

This can all get very complicated and confusing for the average modeller. Unless someone is really interested and wants to delve into the details - which for most hobby modellers is really not necessary - then keep your RX battery maintenance regime simple.

What I find works very well and has caught problems before they've become an issue, are these two workouts for an RX nimh battery........

 

Capacity discharge test using  a regular three button charger/discharger..............Set to cut at 1V/cell and a discharge current set to 0.5 C. Note the readings with a new or known good battery and check for changes in discharge time as the pack ages (usually caused by a single cell in the pack beginning to fail).

Manufacturers' capacity figures are usually optimistic and in my experience can only usually be repeated at very low discharge currents - certainly well below 0.5C

 

Short duration higher load test. Monitor pack voltage with a discharge current of 3C minimum. A freshly charged battery that can't maintain the nominal 1.2V/cell at that load for at least a minute, and better still a couple of minutes should be investigated as either failing, or being of poor quality. 

If a battery can pass this test, then it'll be fine to cope with what'll be asked of it in most sport models, but a one size fits all will be impossible to describe.

 

Two simple tests that can be performed without specialist test gear and only basic knowledge that can be found on the web.

Yes, one can go very much deeper into all this and professionally, I've had past experience of specifying, designing and testing small power supplies for portable industrial test gear back in the day. We very rarely need to go into such detail with our models' power needs but it shouldn't be ignored totally. Standard sport installations are usually perfectly fine with what we have available in the way of nimh packs and BECs from model shops etc. Your regular club hack with four or five standard, ten or fifteen quid servos will be fine on regular AA type RX packs. Start to get away from that with higher performance models with higher power servos - possibly digital - and the attendant extra power requirements will come into play and might catch out the unwary.

 

 

[Nigel R]

22 hours ago, Cuban8 said:

Capacity discharge test using  a regular three button charger/discharger..............Set to cut at 1V/cell and a discharge current set to 0.5 C. Note the readings with a new or known good battery and check for changes in discharge time as the pack ages (usually caused by a single cell in the pack beginning to fail).

Manufacturers' capacity figures are usually optimistic and in my experience can only usually be repeated at very low discharge currents - certainly well below 0.5C

 

Exactly what I was suggesting doing. Apologies if my post wasn't very clear! Although I would use 1C and expect about 80% capacity to show on the discharge cycle.

 

Decent manufacturers do provide discharge curves for various different load currents and they are repeatable. If you go from those, then the figures are not optimistic at all.

 

If you expect the nameplate capacity at max discharge then you will be disappointed, and the datasheets for nimh cells indicate the capacity is nominal at 0.1C (or similar low discharge). I've not seen a credible manufacturer claim anything but this.

Edited October 18, 2023 by Nigel R