Charging Multiple NimH Batteries

[Andy J]

Having now changed all my models over to a mixture of 4.8 and 6.0V nimh batteries I find charging is becoming a right pain.  Principally this is because it takes a considerable time to charge safely a nimh battery, secondly one model contains three batteries and lastly I have only one suitable charger to do the job. Thus flying a selected model has to be prepared well in advance which simply does not work well for me.  My immediate thoughts was thus to buy a couple of extra charges designed for nimh.  Having browsed the second hand market on the web, little seems available other than chargers designed principally for Lipo but also having other battery types as a secondary feature. Few seemed capable of multiple nimh batteries other than the Ripmax ProPeak 4  and none seemed available for under £20. Thus having dismissed the Ripmax charger as it output was 600mA which is quite high for say an 1800mah pack I need to find an alternative solution.

 

Charging these nimh types should be fairly simply requiring only a constant current source at a suitable voltage for a given time so has anyone designed a suitable circuit for this purpose or come up with a way of repurposing a commercial charger?

 

The ideal solution for my set up would be a selectable cell count of 4 or 5 and a variable current between 100 to 300mA with an automated shut off after a given time of say ~14Hrs or delta peak detection.

 

 

 

 

[Martin Harris - Moderator]

The perceived wisdom is that unless you’re prepared to fully discharge and use a timed and fixed rate charge, NiMH batteries should be charged at at least C/2 to avoid false peak detection cutting the charge too soon. 
 

Simply leaving a NiMH on anything above a trickle charge for any extended time after capacity is reached risks damage through overheating and gassing. 
 

Charging on peak detect at or at a little less than the C rate means that your recharge times will be quite short so that would help address your problem. 
 

One tip would be to ensure that multiple peak detect charging in a model can only be done with the packs fully disconnected or using separate power supplies to the chargers.  The common “earth” connections on a standard installation confuse intelligent chargers otherwise. 

[RottenRow]

Andy,

 

A use the old SM Services Multi Charger.... these have six separate outputs, each selectable between 11mA and 220mA (in steps). Each output times out after 14 hours, and the charging LED then flashes to indicate this. Charging at, or about, 0.1C for 14 hours is fine for the batteries irrespective of their initial state of charge.

 

There is also a constant voltage output, switchable 2V, 6V or 12V, for lead acid batteries.

 

These have been out of production for many years now but they do pop up now and again at swap meets, on the BMFA classifieds, etc. I have several, and most only cost me around £10 each. They should come with a separate 24V DC power supply. The channels can be adapted to give a higher current output (by substituting a resistor), I have done this to give a little over 300mA on a couple of channels of a friend's unit, but anything over that will require some heatsinks to be fitted to the ICs and some extra cooling for the case.

 

Here is the user instruction sheet for the charger, so you can see if it suits your purpose.

 

SM Services Multi Charger MK2.pdf

 

Alternatively, if you wanted to build something yourself, a simple constant current supply based around a voltage regulator such as an LM317T works well enough... there are lots of similar circuits to be found on the internet.

 

Constant current chargers don't need the cell count selecting.

 

Brian.

[Frank Skilbeck]

Andy what where you using before you changed over to Nimhs?

 

BTW there's a Schulze dual output charger on the BMFA for £15 at the moment.

Edited October 18, 2022 by Frank Skilbeck

[Andy J]

29 minutes ago, Frank Skilbeck said:

Andy what where you using before you changed over to Nimhs?

 

BTW there's a Schulze dual output charger on the BMFA for £15 at the moment.

Frank, I was using 11.2v or 7.4v lipo''s and switch mode regulators but found these introduce a high level of electrical noise on the Rx supply lines.

[Frank Skilbeck]

Andy- do the chargers you used for those not have a Nimh setting, I charge all my Nimhs using my multifunction chargers, typically at 0.5c so it doesn't take too long. With a periodic cycling charge just to check their condition.

[Phil Green]

Check if your rx & servos will accept a 2S LiFePo4, 6v6 nominal but can be 7v2 immediately off charge 🙂

All the low-self-discharge benefits of lipo at a lower voltage and minimal risk of fire

[Nigel R]

My nimh charger consists of a lead with a big resistor. I used the same power supply as my lipo charger. No need to complicate. Job done.

 

On a 15v supply you need to drop around 10v. Formula is R=V/I. This gives a resistance of around 68ohm for around 150mA. You need a beefy resistor, say 5W.

 

Total parts list, one long extension lead, one resistor, one pair jack plugs. 

 

Edit: I do an occasional cycle on the multi function charger to check capacity and condition. 

Edited October 19, 2022 by Nigel R

[Paul De Tourtoulon]

I have a couple of the old Robbe delta peak chargers, ( infinity ) the last one picked up for 30€, you can do what you like with them,  as an afternoon flyer I put what I have decided to fly at around 12 noon in the car and put it on automatic leaving at 14h and a 30 min trip to the field they are fully charged,with exception of those silly cheap AA so called 2.400mah supermarket battery's,  but they don't do  lipo's,,,

 

https://www.ebay.fr/itm/114594372654

[John Lee]

19 hours ago, RottenRow said:

Andy,

 

A use the old SM Services Multi Charger.... these have six separate outputs, each selectable between 11mA and 220mA (in steps). Each output times out after 14 hours, and the charging LED then flashes to indicate this. Charging at, or about, 0.1C for 14 hours is fine for the batteries irrespective of their initial state of charge.

 

There is also a constant voltage output, switchable 2V, 6V or 12V, for lead acid batteries.

 

These have been out of production for many years now but they do pop up now and again at swap meets, on the BMFA classifieds, etc. I have several, and most only cost me around £10 each. They should come with a separate 24V DC power supply. The channels can be adapted to give a higher current output (by substituting a resistor), I have done this to give a little over 300mA on a couple of channels of a friend's unit, but anything over that will require some heatsinks to be fitted to the ICs and some extra cooling for the case.

 

Seconded.

 

This is mine with the power unit on the right and the optional cycling unit & capacity checker. Well worth keeping an eye out for one or putting out a 'Wanted' ad. There must be dozens of them lying unused and if mine is anything to go by they are pretty bullet proof.

 

Edited October 19, 2022 by John Lee
Typo

[Andy J]

Reread the Battery University pages regarding charging nimh. Would seem trickle charging is not the way to go so will have to re-think my approach in the future as normally would use a charge current ~300mA over approx 8hrs.

 

Given I had one of my aircraft on the bench for some re-work this afternoon  I thought I would experiment with a higher current setting on my current intelligent charger. Battery selected was a 4.8v 3300mah which is positioned within a part of the fuselage that allowed at least the skin temperature of the battery to be monitored by a calibrated finger. First I disconnected it from the model to avoid the charger miss detecting the delta peak and then setting a charge current of 3A. Certainly after approx 1 1/2 hrs the battery felt warm which I know is not good, so in future I will select a lower amperage. On the positive side the battery voltage increased from 5.2v to just shy of 6.0v however the charger was continuing to push out 3A  so would appear that there is still some more capacity to be had before the delta peak is detected.

Will continue to charge the battery tomorrow at a setting of 2A  to see if that is a better approach to reducing the charge time whilst avoiding the battery getting warm.

 

Have purchased a second lipo charger as a further back-up but certainly the SM service charger could be a good option if one becomes available.

[Richard Wills 2]

It is quite normal for a nimh cell to reach 55 deg C towards the end of  a 1c+ fast charge, your calibrated finger will perceive this as red hot.

[Dickw]

3 hours ago, Richard Wills 2 said:

It is quite normal for a nimh cell to reach 55 deg C towards the end of  a 1c+ fast charge, your calibrated finger will perceive this as red hot.

Agreed. Before switching to Lipo to power electric models we used to recharge nimh cells at 1 or even 2C between flights. They did get very warm, but that wasn't seen as a problem. Some chargers offered a temperature cut-off as an alternative to voltage peak detect. 

 

Dick

[John Lee]

15 hours ago, Andy Joyce said:

Reread the Battery University pages regarding charging nimh. Would seem trickle charging is not the way to go so will have to re-think my approach in the future

Likewise I would not get too hung up about trickle charging.

 

The memory affect can be overcome by the occasional cycling of the battery.

 

Otherwise a practical example of trickle charging in practice is the system used at the RCHotel for the last 20 years. There you plug the Tx's & Rx NiMh's into trickle chargers immediately after each flight where they remain 24/7 throughout the season. Rx NiMh's were checked before each flight with a HiTech Power Mate battery checker which puts a load on the battery (hardly used now that their fleet is 95% electric). And of course the Tx battery as part of your SMART checks. Whilst this is defiantly sub-optimal for the batteries it does work and makes commercial sense in that environment. Battery failures are rare, they have occurred but should be picked up by the checks.

 

I'm not saying that you should follow this practice, but would advocate that you don't need to strive for perfection. NiMh batteries are pretty tough.

[Andy J]

20 hours ago, Richard Wills 2 said:

It is quite normal for a nimh cell to reach 55 deg C towards the end of  a 1c+ fast charge, your calibrated finger will perceive this as red hot.

Disagree Richard, if a battery is getting hot then this will shorten its life.  Think the reason for your battery getting hot is likely your charger is continuing to charge at the selected current for the duration of the charge.  Believe the optimum for nimh is to charge initially at 1C and then slowly drop the current thus avoiding the overheat issue.

 

Certainly charging two 3300mah 4.8V batteries today at 2A showed no sign of temperature increase with the charger successfully detecting the delta peak and terminating the charge.

[Frank Skilbeck]

Andy, not quite, for Nimh you need to make sure that the small voltage drop when the battery is full is detected and the charge is stopped. Some info here 

[Richard Wills 2]

43 minutes ago, Andy Joyce said:

Disagree Richard, if a battery is getting hot then this will shorten its life.  Think the reason for your battery getting hot is likely your charger is continuing to charge at the selected current for the duration of the charge.  Believe the optimum for nimh is to charge initially at 1C and then slowly drop the current thus avoiding the overheat issue.

 

Certainly charging two 3300mah 4.8V batteries today at 2A showed no sign of temperature increase with the charger successfully detecting the delta peak and terminating the charge.

I should hope it would continue to charge at the selected current to the end of the charge, that is what I asked it to do afterall.  If the current drops towards the end, you risk a false peak and the battery not being fully charged.  I do not consider this to be an overheat issue, rather it is expected, and normal.  More than one way to skin a cat, and we all do what we are comfortable with.

 

[Andy J]

Should say that whilst my intelligent charger is selected at 2A this is not the current that is used in auto charge mode. Current applied is continually changed with time ramping both up and down with brief periods of charge current removal which I assume is to see what the battery voltage settles at.  Unfortunately I have no details of what algorithm it is actually using in Auto mode but at least not allowing the charge to be above 1C I am avoiding the battery getting hot.

[Richard Wills 2]

mine does a similar thing, I assume it is like the step-differential charge mentioned on the above linked page.  It bulk charges until I would expect it to be nearing full, then cuts off periodically for a moment then ramps back up.  this repeats and it cuts off usually as it is ramping up after one of the pauses.

 

[Nigel R]

On 19/10/2022 at 17:44, Andy Joyce said:

Would seem trickle charging is not the way to go

 

If I'm honest, I think you may misunderstand the page. It doesn't say "don't trickle" or "trickle is bad for nimh".

 

Quote

Charging these nimh types should be fairly simply requiring only a constant current source at a suitable voltage for a given time so has anyone designed a suitable circuit for this purpose

 

I think you're overthinking this.

 

At the risk of repeating myself, the 'suitable circuit' really is as simple as:

 

A 15V power supply from your fancy lipo charger. Or a 4s Lipo. Or a car battery. Or a laptop supply. Whatever.

One dropper resistor.

The RX pack.

 

Leave on overnight.

 

If you are concerned about heating effects, pick a resistor that gives a charge current of 0.05C, or, around 100mA (assuming a regular Eneloop with 2000mAh), and leave on for longer. That's about 100Ohm. Heck, use a slightly larger resistor and charge a bit slower if you want.

 

Sure, you can mess about with an LM317 or whatever, with heatsinks and a few resistors to set the current and a couple of caps and so forth, but why bother. These dropper leads are so easy to make, you could knock up enough of them to cover your whole fleet in half an hour for under a tenner.

 

And the last bit is, if you repeat this regime every month, the eneloop (or other low discharge) nimhs will lose so little charge, they will be ready to go whenever you are.