D-cell Ni-Mh 12v battery-pack (10S) ok for a 12v device that can be a little heavier/bulkier?

Question

In a previous question I was asking about a way to use dc charging in combo with an 18650 battery pack for an audiobox. Since site rules mean I cannot change or add to that question after it has been answered, this question is about using a different (Ni-Mh) battery pack instead of Li-ion for the same project.

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The project is a Bluetooth audio front module built into a box with small speakers and a battery solution to power it beside a normal dc connection. The BT front has a 2*25w amp that draws between 1-3A at 6-12v. Ideally, a 12v battery will fully power the amp.

I've been looking into the popular 18650 option, but since using a normal DC-socket and charger is turning out to be quite cumbersome to achieve and clear-cut guidance into doing so is hard to find online, I've been looking into other kinds of battery-options, other than Lithium, as well. And I have another question pertaining to it, at least if the admin(s) do not mind?

I should note that the audiobox is not really meant to be "portable" like a little BT speaker, I only want to be able to pick it up and put it in my car to use at other fixed locations, and I do not mind at all if it weighs a few pounds. This is why I'm also looking into alternative batteries to Lithium, and for my intent, rechargable D-cell batteries do actually seem like a viable alternative.

D-cells are available at fair capacities of 5-6000mah or more, and although I'd need 10 of them in series to have 10x1.2v=12v, their capacity would be close or even better to the 6000mah I'd get from the 3s2p 18650's. Here are 5700mah specimens from a shop I know to be reputable with certified products. I'm from Europe btw so I mainly use European shops, except for ebay and aliexpress for knobs and bobs.

https://www.123accu.nl/GP-ReCyko-Oplaadbare-D-HR20-Ni-Mh-Batterijen-2-stuks-5700-mAh-i20290.html

D-cells also hold their stored energy for extended periods which is ideal since the device would not be used on a daily basis either. I could just make the box with an extra bottom compartment to fit them, and the base of the box would have a little more than the squared dimensions of the 10 D-cells side-by-side. They are a bit expensive at 100 euros for 10 pieces, but considering the limited use of the device once built, these could last for many years without issues.

Looking at it, it seems a lot safer and less hassle than Lithium and I can use any 12v charger to charge them. I take it they can be placed in series the same as 18650's ... But do they still need a BMS or some kind of protection circuit and/or balancing, or do I just have to make sure they are all the same specs and at same charge-level?

I could make it so that they can be removed from the bottom like any commercial device usually has, and charge them externally, or I could have them charged internally via a DC socket?

What do you think, for an audio box that isn't used daily and remains just sitting shut down for periods, and just needs 12v at 5000+ capacity? Anything other I would need to take into consideration for Nimh batteries or how to charge them?

Answer 1

It depends how long you need to power it outside the car. My choice would be ten of the far smaller, less expensive AA-size NiMH cells for perhaps 1/2 hour play unplugged -- but it should run continuously when plugged into a 12 VDC car "lighter" socket. The car battery can provide about 9 VDC (while starting) to 14 VDC (fully charged), but most of the time would be ~12.6-13.7 VDC, which provides just the right charging voltage for ten NiMH cells.

Add a "polyfuse", i.e., a resettable fuse in series with the battery, to prevent damage if it were accidentally shorted or reverse-connected. Digikey, for example, lists many resettable fuses in the 21-30 VDC, 2-3A hold current, range.

If you want to know the state of charge, add a digital panel voltmeter, such as this from Amazon (~US$3 each) and a switch, so as not to discharge the battery when not in use (or use the circuit power switch). Unplug charger if voltage is over ~13.9 volts.

IMHO, NiMH cells are easier to apply in circuits then Li-ion, though heavier for the same power, as you state.

• Some types of NiMH have a catalyst to prevent damage for very mild overcharging by recombining evolved hydrogen and oxygen.
• Those cells work well in series, needing no balancing device.
• The chance of fire is greatly reduced (though the battery can provide enough power to start a fire in other material, if shorted... so, use a fuse).

Answer 2

You basically have the same problem with all battery technologies. It does not matter what batteries you use, you need to charge them properly, make sure cellls are balanced enough, and be able to automatically cut off when voltage is critically low.

Another problem is wanting to charge the batteries inside the device, from a DC plug. As DC plugs are (generally) used for providing supply voltage, devices with DC plugs are not chargers but voltage supplies. Therefore, you need to integrate the charger too. The DC socket is not required to directly go to battery, and maybe it should not either, to prevent shorting things out. Which is why fuses are mandatory.

Just like laptops take roughly 20V in or mobile phones take about 12V in, they have a charger inside the laptop for the pack they use, regardless of what kind of pack is in there. So they include a charger that can take some range of voltages in and convert the voltage to charge the battery as per their chemistries.

So you can implement that too - only that you seem to want to buy these functionalities as ready-made modules, and thus due to the site rules we cannot tell you what modules to buy and from where, if they even exist.

Having said that, how about using a 12V lead-acid battery instead. Not the ones used to start combustion engines, but the deep-cycle types, used in UPSes etc. They are much simpler to charge and manage, also you can find chargers for them so your device could have a mains inlet for charging, or even some 14V-20V DC inlet, or if you find a charger like for RVs that take in 12V from car main battery side to charge the auxiliary battery. Such batteries and chargers may be found from places that have this kind of equipment for RVs, boats, etc.