Battery substituting adapter circuit, initial ideas - Page 1
Forum section: electronics / projects

author: Infraviolet
author_rank: Super Contributor
timestamp: January 03, 2025, 08:10:13 pm
content: Has anyone any initial ideas on roughly what sort of "blocks" I would have to lay out, in terms of sections of a circuit performing various functions, so as to design a plug in battery "module" which could be used to fit a different battery in to an existing device. This is a very early stage concept for a project I'm thinking about, and I'm just trying to get a feel for what will be involved.I'm talking about situations where you want to replace a LiPo cell with the usual 3 terminals (+V, Gnd and one for temperature/insertion detection) with a module containing an array (for much larger capacity) of multiple cells (which might be of different chemistry like NiMH instead). And you still want this new module to be able to be charged via the device's existing charging circuitry.So you'd be looking at something where your module would have to have an output which could emulate the voltage output of a LiPo (obviously using a different overall capacity so you'd discharge along the curve over a rather slower time) when feeding power in to the device (any manner of portable gadget, up to and including something like a phone or laptop), but your module would also have to be able to absorb power (in a manner similar to how a LiPo single cell would) when being charged from the device (we're considering only scenarios here where the device itself contains the charging circuitry so during recharging it is simply being fed fixed voltage DC from a wall wart which it is then converting interally in to the "signal" needed to charge a battery).Internally the module could, when dischrging to power the device, have a bunch of series cells inside it, feeding to a step down buck converter to give a voltage output within the range expected from a single LiPo. It would need also some way of making this converter decrease slightly in output voltage when it detected that the stacked series of cells inside it was getting towards their low voltage cutoff (so that the device's own circuitry would recognise that the battery needed charging).I'm much more confused about how you could have a circuit which would emulate the getting-recharged profile of a LiPo cell, and then convert this to a higher voltage which would be used in a charging circuit to fill up the series cells.Can anyone suggest:1. If there's a simpler way of handling this concept than having whole separate circuit "blocks" which would work in both directions to take in varying voltages, convert them to a stable regulated level then convert this again to a variable output. And therefore whether the architecture of the system I've got in mind here is the right way to approach this sort of problem (creating alternative larger capacity battery packs, preferably in NiMH where a mistake doesn't lead to flames as it will with LiPo)?2. What separate circuit elements would need to be designed: "an NiMH series charging from regulated DC unit", "a unit able to take in the voltage/current form which charges a LiPo and provide a regulated higher DC voltage from it", "a buck converter able to take varying higher voltage inputs and output a voltage which can be varied when needed to look like a LiPo nearing end of discharge". And where I can find good sources with various examples of circuit subsystems for these purposes?I've never really done much detailed work with batteries before, simply stacking them in series and having a ADC monitor the voltage to know when the per-cell cutoff is reached, and then using pre-built wall-wart charger units to recharge them, so having some ideas of good places to read up about those would be really helpful too.Thank you

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 03, 2025, 09:04:34 pm
content: Using the original charging circuit will provide the same current, thus the charging will take (New capacity) / (Original capacity) times.If you plan to put a much larger battery (I.e. 10x larger), it could take even several days to fully charge.But, otherwise, swapping the existing battery with a much larger one, same chemistry, should be almost a direct replacement.The temp. terminal is often a simple NTC

author: Salitronic
author_rank: Contributor
timestamp: January 03, 2025, 09:57:21 pm
content: One issue to consider when emulating a different battery through some form of step-up or step-down conversion is the conversion and quiescent losses, especially during standby. If you have a device that is on standby most of the time, a good design with the original battery might easily achieve a few microamps of standby current, possibly less. With your battery emulator using different battery chemistry, it would have to constantly boost the voltage to maintain the expected voltage, wasting much more than those few microamps of the original battery. Despite having a larger capacity, you might end up just wasting energy.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 09, 2025, 07:33:29 pm
content: Thanks for the tips, that note about much slower charging of a bigger battery when using the existing circuitry of the device makes good sense. Interesting thought about efficiency losses too, though with a sufficiently big new battery that sort of thing might not really impact too much on times between needing to charge.If one were to consider charging such an added battery via its own dedicated power input instead then... what would one have to do to stop any problems that could occur if the device tried to "back charge" the output end of the added battery's step down converter? That is to say, imagine you've got something like a phone or camera which both transfers data and draws power to charge itself over a USB cable to a PC. If one is instead charging a newly added battery pack via a separate (possibly 12V instead) DC input, then one does one have to do in the area of the circuit where the buck converter has its output so as to avoid causing any actual problems when the device, whilst transferring data over USB, attempts to pump charge it to what it thinks is a LiPo battery, but which in reality has been replaced by the output of a step down switching regulator?

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 10, 2025, 12:31:56 am
content: They'll work in parallel, shouldn't be a real issue.

author: inse
author_rank: Super Contributor
timestamp: January 10, 2025, 06:57:55 am
content: I swapped the 4xC NiCd battery pack of an older device with a USB power bank I had laying around.Made a hole in the housing to charge via USB externally.The lifetime is so long now, I haven’t even recharged yet…

author: Infraviolet
author_rank: Super Contributor
timestamp: January 13, 2025, 06:29:22 pm
content: "They'll work in parallel, shouldn't be a real issue."Really?If you've replaced an existing battery with something which isn't actually a battery but is rather a battery pack who's output is the output of a buck converter or other step down regulator then you end up with a situation where the battery pack you've built is trying to output through the buck converter's output, but at the same time the device is trying to shove power in to the output of the buck converter.I can't call what follows a "formal" finding, but I once found myself in a situation like this where I had built a robot's circuit board which featured a raspberry Pi. In use this circuit board powered the Pi via the Pi's 5V pins, the circuit board was powered by a pololu branded 5V step down regulator, with an input fed by a battery. One time I accidentally powered the thing by plugging a USB power lead in to the Pi, so the output of the buck regulator was being "back powered" (if that's the right word). I saw and heard sparks around the regulator, pulled the USB lead immediately and nothing seemed permanently damaged when I powered it properly via the battery supply again later.But if a step down regulator's output can spark when you try to supply power in to its output then I can't imagine that it can be very healthy if you modify a device in the fashion I've been describing without putting some sort of diode (or other one way circuitry, I'm asking here because I'm not sure what form it should take) system between the output of your high voltage battery series's buck regulator and the pin on the device which would usually serve as the battery positive terminal (a terminal which takes power from the battery when discharging, but also pumps power in to it when the device is being powered from a wall wart and is recharging its battery (or attempting to recharge a battery which is no longer there since you substituted it with a pack which has a regulator as its output).Thanks