Replacing a NiMH battery pack with LiPo battery - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: May 29, 2022, 09:31:15 pm
content: I have a device that uses a 4.8V NiMH battery pack of approx. 300mAh capacity, formed of 4S 1/2AA-size cells, but it's dead and I want to try and replace it with something with more capacity, as the 300mAh was never really satisfactory anyway. I'm thinking of possibly a LiPo battery.I found that I can get a 1400mAh LiPo pouch-style battery that is roughly the same dimensions (NiMH: 14x30x56mm; LiPo: 12x30x40mm), but I'm not sure if I can (or should) use it as a replacement.The device in question has a solar panel to charge the battery (which outputs about 8V) that is permanently connected in parallel with a series arrangement of a diode and the NiMH battery, regardless of the on/off switch state. If the LiPo battery in question has a built-in protection circuit, is it possible to connect directly just like the NiMH battery was? As I understand it, that protection circuit should prevent over-charging and do under-volt cut-off, but probably won't do charge current limiting. But I doubt the latter is actually necessary, given the small solar panel cannot provide much current in the first place.The voltage difference (3.7V vs. 4.8V) shouldn't in reality matter, as of course the 4.8V is the maximum for the NiMH (4x 1.2V), whereas the 3.7V LiPo is its nominal rating, with a maximum of 4.2V. The internal circuitry of the device runs off a 3V linear regulator, so both batteries have similarly satisfactory discharged-state voltages.What about operating temperatures? It is a concern, as this device operates outdoors. The LiPo battery says it is rated for 0-45°C, which should be okay (it'll never be operated in winter), but do LiPo batteries suffer more from temperature extremes than NiMH? I can imagine it might get near that upper limit in direct summer sunshine.So, should I swap NiMH for a LiPo?

author: HwAoRrDk
author_rank: Super Contributor
timestamp: May 31, 2022, 05:22:03 pm
content: I think I had a misunderstanding about Lithium battery charging voltages. It seems I don't want to apply any charging voltage above 4.2V, so taking 8V straight from the solar panel will be a no-no, even if it's at a very low current. And thinking about it, the LiPo cell protection circuit will obviously cut off straight away when it sees 8V.I'm thinking instead I should just grab one of those cheap TP4056 charging boards, connect that to the battery and disconnect the solar panel entirely. I can probably modify the device with a barrel jack or something to have a charging port. Having a much larger capacity battery will at least go some way to mitigate having to manually charge the device every now and then, as opposed to trickle charging from the solar panel.

author: tunk
author_rank: Super Contributor
timestamp: May 31, 2022, 09:09:08 pm
content: AFAIK all solar panels have voltage vs. current curves for for given solar intensities.And 8V is under ideal conditions, i.e. the highest solar intensity and with little or noload. So your cell will probably never see 8V. I think NiMhs can take 0.05C tricklecharge, in this case 15mA at around 6.2V. I wouldn't know what it can deliver at 4.2V,but would guess it's far below 1C (in this case 1.4A), so I would think that it wouldwork with a Li-Po cell assuming the BMS will cut off charging.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 31, 2022, 09:43:38 pm
content: What you'll probably end up wanting is switching regulators to take variable solar cell input voltage and give a constant voltage to a charge controller IC, and regulators to give a constant output voltage when you draw from the battery.But also, you could consider using some much bigger NiMH cells (might be a mechanical fit nghtmare, but as voltages would be the same I suspect minimal electrical modifications needed?). You said there were 4 in series, in series you don't add the mAh of cells, you can buy AA NiMh cells with >2500mAh to get 2500mAh overall, or D cells with as high as 10000mAh. I don't know what the device is but could you manufacture some sort of big adapter casing to hold a bank of 4 higher capacity NiMH cells and simply run wires from this in to the old batetry compartment via some reasonably well sealed method?

author: edavid
author_rank: Super Contributor
timestamp: May 31, 2022, 10:04:23 pm
content: You could run the TP4056 from the solar panel if you add a shunt regulator to avoid exceeding its 8V max input voltage, but there are plenty of similar ICs with a higher limit.How about the BQ2057?https://www.ti.com/product/BQ2057

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2022, 11:55:41 pm
content: I would not use a LiPo, at least not one that lacks a built in protection circuit. LiPo batteries are notorious for catching fire if they get overcharged.

author: thm_w
author_rank: Super Contributor
timestamp: May 31, 2022, 11:59:13 pm
content: TP5100 is another alternative to BQ2057:https://www.aliexpress.com/item/2251832793103609.htmlIt can do 4 or 8V output. Input voltage up to 18V.But if OP had a TP4056 on hand, considering the very small size of the pack, yeah could just throw in a LDO or shunt.

author: BeBuLamar
author_rank: Super Contributor
timestamp: June 01, 2022, 11:51:36 am
content: You can keep using NiMH. Newer cells of same size generally available with higher capacity.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: June 01, 2022, 05:18:18 pm
content: The 8V from the solar panel was what I simply measured with it pointing at a window and no load, so I thought I'd better try it with. Seems the output voltage does indeed drop a fair bit with some load. If I add a 1K load, then it gives 3.5V, albeit it is quite overcast today, so that is probably nowhere near the full potential of the panel. A paltry 3.5mA isn't going to charge a battery very fast, and I suspect the charge status LED on a TP4056 board would probably sap all of that...Maybe I will indeed forget the solar charging and just bodge in a charge jack.Quote from: Infraviolet on May 31, 2022, 09:43:38 pmI did actually previously have another of these devices where I had modified it to have an external battery pack consisting of 4xAA NiMH batteries, but I could never get the battery pack sealed properly against the weather. I'd prefer this time to keep the battery internal.

author: edavid
author_rank: Super Contributor
timestamp: June 01, 2022, 06:26:42 pm
content: Quote from: HwAoRrDk on June 01, 2022, 05:18:18 pmBut how would you waterproof that?Is there room inside for a Qi charging coil?