Suggestions for 24V Battery Pack? 20s NiMh?

[Magnum]

Hello,
I am building a portable test system which needs 24V battery power (to be exact: between 20 and 30V). I don't need a high capacity, so 2-3000mAh are sufficient.

I was looking to find something assembled, but didn't find something suitable. So I built a 7s LiIon pack with a BMS. It works, but I don't have much experience with LiIon. To load the pack I have a 24V power supply installed, it is adjusted at 28,7V as I don't want to overload the cells (even with the BMS). But I don't know if that is so good for the cells when the power supply is connected for a long time.

Now I am thinking about using 20s NiMh cells, as they don't tend to catch fire so easily, and I don't need the energy density of LiIon. Do I need a special battery loader to reload them?  I think they don't like to be overloaded? Is it possible to just use a 24V power supply (maybe adjusted to 28V)? Are there BMS for NiMh available?
Or does someone have a better solution?
Many thanks,

Magnus

[penfold]

Basically, if you're intending to be charging often and regularly, I would recommend a dedicated charger. NiMh is reasonably forgiving to errors in charge voltage or current at first but do benefit from a proper temperature regulated charge cycle if its a regular thing. During discharge NiMh is significantly more stable than your average lithium-ion so BMS isn't essential. The best use case for NiMh tends to be in backup power for low power demands where their charge is maintained for long periods of time and have very few discharge/charge cycles.

It may be worth looking towards a LiFePO4 option, they aren't as prone to fire as most lithium-ion chemistries and honestly are probably more suited to your application than NiMh.

Could you possibly get away with a lower battery voltage and step-up regulator?

[nightfire]

For portable solutions lots of people have made good experience with standard LSD batteries like the Eneloop series (now Fujitsu FDK that seems to be preferred).
Here a good quality standard charger is more than sufficient, I personally have an old MEC AV4m (that also gets rebranded like from IVT) and I am happy. When you need to use more than 8 batteries, a charger with more than 4 slots is probably nice to have.

Forum member HKJ has some nice comparison/reviews on his page: http://www.lygte-info.dk/

As  NimH are also useful in lots of other appliances, I would consider it a safe bet.

Another go at 24V could be a standard Lead Battery (2x12V), that type that comes sealed- here it is nice to have a special charger available that can deal with lead cells, or a lab PSU that also offers some funcionality regarding this...

[richard.cs]

If physical size is not too big an issue for you, you could use surplus Clansman military radio batteries. Last time I bought some they were about £15 each, and you get 20 NiCad D cells in series in a nice, super-robust case. The more recent ones are around 5 Ah. Being NiCad charging is easy, you can trickle charge them at C/10 forever without damage.

Power tool batteries are another option, there are 24 V ones out there and you can buy them with a charger and all the battery management sorted for you.

[Magnum]

Many thanks for all the replies!

What I forgot to mention: The charger should be installed in the test system. Right now I am using the 24V power supply to charge the batteries AND power the system. If I don't have ac power available, the system runs from the batteries. That is why I am looking for a battery that is easy to charge.
My system works between 20 and 30V, so I would like to set the power supply to a voltage that won't overload the batteries. The Power Supply has 2 Amps, so if I use 2500 mAh Batteries the charging current is less than 1C. When the system is powered up, the charging current will be less.
Does it harm NiMh batteries if they are loaded only up to lets say 90% of maximum voltage? What happens if they are loaded for a long time (20-30hours)? I don't care if they loose some capacity, as 2500mAh is more than I need.

[nightfire]

Depending on your price setpoint, I would look at industrial solutions. Lead/Acid AGM batteries are standard for lots of applications like buffering burglar alarms in bigger setups, so there are lots of (certified) charging solutions available. With regular maintenance those batteries should be exchanged every 3 years, and you are set.

[langwadt]

get a 20V capable USB-C powerbank and an PD adapter to get 20V ?

[coppercone2]

"NiMH dislikes overcharge, and the trickle charge is set to around 0.05C. NiCd is better at absorbing overcharge and the original NiCd chargers had a trickle charge of 0.1C. The differences in trickle charge current and the need for more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger functions well. Modern chargers accommodate both battery systems.

It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold. "

https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

So if you have unmatched batteries, you can charge it at 0.05 C for a long time in a string, I think, if you cannot get sense lines on them. I think thats OK, I like NIMH too for things like lab amplifiers and weird shit, don't feel like dealing with lithium there.. this is easier. But I just take them out of the circuit and charge them on a battery charger, instead of charging in circuit. 20S is a little bit hard core for D cells.


Here is a 'field report' about what you want to do. I personally have to charge 9X d Cells and 8X C cells and I don't wanna take the screws out of my battery boxes, so I am doing research.
https://www.candlepowerforums.com/vb/showthread.php?239572-Charging-12-NiMh-Batteries-in-Series

I share your belief that all the people saying you should integrate charge controllers and other stuff into simple projects are pretty crazy (every battery thread ever would have you double or triple costs to make it plug into something). I recommend investing in quality battery packs that don't break with removing cells lol.

I am trying 1/20C on a string of 10 D cells right now.

Here is some real info, its not THAT scary like people make it out, panasonic lists it as an option
https://www.epectec.com/downloads/Panasonic-NiMH-Cell-Lineup.pdf


My guess is that you will get like, slightly more life time out of the batteries you go all out to keep charged (10% ? ) vs the ones you series slow charge. For me that is good enough.

[tunk]

How much power does your system draw?
Depending on how much its draws, langwadt's suggestion could work.
Also remember that a fully charged NiMH is 1.4-1.5V, so 24V will not fully charge your battery.

[wizard69]

The suggestion for lead acid batteries is a good one.   You want sealed solutions and probably batteries specific for any orientation.  Batteries like these: https://www.digikey.com/htmldatasheets/production/474466/0/0/1/cyclon-reg-selection-guide.html get used in a variety of backup applications.   With 2 volt cells you can build up the battery voltage that suits the application.   You might think that lead acid is old tech and in a way it is but you can also get proven support hardware such as chargers.    Even simple DIY chargers are possible.

The only other suggestion that seems viable here would be the Lithium Iron Phosphate batteries.   This gets a lot more complex though when it comes to charging and battery management. 

By the way somebody mentioned above using power tool batteries.   This isn't a bad idea but in your case you seem to want a built in solution.   However as pointed out all of your engineering is solved for you.   

The other issue here is that we don't know a lot about your test rig.   It might make sense to implement a lower voltage battery and step the voltage up when running on battery.    This mainly due to weight as even Lithium will add nontrivial mass to your test rig.    Of course operating time might be an issue but those are details we don't have.   In any event there are many prepackaged 12 VDC solutions with all sorts of chemistries that could save you lots of work.   12 VDC due to the automotive world.   Going this route I'd look for battery suppliers and suppliers of charge control circuitry online.   

[Magnum]

Many thanks to all for the great suggestions.

I think I'll go with lead acid or with the Lithium Iron Sulphate. I'll order both and try them out.

Again, many thanks.