Best way to hack a 6x NiMH charger into a musical keyboard? - Page 1
Forum section: electronics / projects

author: Freesurfer
author_rank: Regular Contributor
timestamp: August 26, 2021, 07:38:38 pm
content: I have a Yamaha keyboard that takes 6x AA batteries. It can also be run with a 12V DC adapter. But it does not charge the batteries from the DC adapter.I want to know if I stick in 6x NiMHs if there is some easy way to hack in a small charging IC or other circuit that can charge the batteries while the DC adapter is plugged in. Something that can be soldered in between the DC jack and the battery compartment, perhaps with a small switch to disable charging for alkaline batteries.The charging does not need to be fast. It is plugged in most of the time. it's just so it can run hassle free on battery power whenever unplugged. And it does not need to be super accurate either, as long as it doesn't ruin stuff. There are no mid-point connections between each cell, so needs to handle 6 batteries in series without individual cell balancing. If the current is small enough, can NiMHs be overcharged indefinitely without being damaged?

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 27, 2021, 02:11:17 am
content: Base it on this design:http://www.ultrasmartcharger.com/Scale it up to 6x the voltage.

author: bob91343
author_rank: Super Contributor
timestamp: August 27, 2021, 05:07:25 am
content: I did that very thing to an old boom box. All it took was one resistor and maybe a diode across the external dc jack.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2021, 06:22:34 am
content: First, confirm it can run properly with NiMH batteries. Depending on its minimum operating voltage, it may not be practical to use rechargeables in it. If it will run OK down to 1.1V/cell (6.6V at its battery terminals) you are golden, but if it requires more, only a fraction of the nominal battery capacity will be usable (and it will already be a battery hog when using alkalines).Overcharging NiMH cells is highly undesirable, and if sustained, frequently results in cell damage and venting, with consequent loss of electrolyte and possible leakage (which if it does occur, is of a highly caustic alkali, and is at least as damaging as alkaline electrolyte). Trickle charging must always be below C/40, and lower is preferable. I doubt you can tolerate it needing two full days to recharge! However Mike has already pointed you at an open hardware smart charger design so 'dumb' charging issues can be avoided.Over-discharge is far more of a problem. The cell capacities wont be equal, so unless you use an undervoltage cutout circuit to protect them, there is a risk of cell polarity reversal of the weakest cell during discharge if you take the battery past the knee of its discharge characteristic, which will rapidly damage that cell, then the next weakest until half the cells are bad!TLDR: if this was easy, Yamaha would have sold it with a charging circuit and an option for a rechargeable pack to fit the battery compartment!

author: Kjelt
author_rank: Super Contributor
timestamp: August 27, 2021, 06:52:47 am
content: What I sometimes do is solder a batterypack lead (Tamiya or BEC depending on current) to the cells which also need to be soldered together. Then the contra connector to the equipment and plug them together outside of the machine ( if you have room and a sturdy batterycompartment lid you can put them inside ofcourse)When it is time to charge I use a fast intelligent charger to charge them in two hours.I just did this last week to a 90s meauring device costing new over $5k because the internal charger was crap, its transistor and coolplate went over 80oC within ten minutes.

author: Freesurfer
author_rank: Regular Contributor
timestamp: August 28, 2021, 09:22:51 am
content: I actually won't mind charging as slow as 40C if that's what it takes to simplify the circuit to the absolute minimum. The issue I am having is that there is no indication whether the keyboard runs on batteries or the DC adapter. It turns on and plays normally. But if I unplug and forget about it, the batteries run flat before I realise it has been sitting unplugged all the time.. so then it takes another pack of fresh batteries. But if instead I can stick a bunch of rechargeable ones that the keyboard will take care of by itself then that's great. Even if it takes 2 days. If I need faster charging I can just charge them manually.It runs fine on 7.2V and the manual even states that it can run on alkaline, manganese and NiMHs.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2021, 09:52:50 am
content: NiMH without an undervoltage cutout really wont like that usage pattern. Running a multi-cell pack till dead is guaranteed failure due to reversal in very few charge/discharge cycles. If you added beefy antiparallel Schottky diodes across each individual cell to prevent significant reversal, it might last a bit longer, but they'd be a significant fire hazard if a cell was misinserted so that's only suitable for hard-wired packs built of tagged cells.Since the consequences of disconnecting it seem to be flat batteries anyway, why not keep no batteries in it and only fit them when you need to use it portably?

author: magic
author_rank: Super Contributor
timestamp: August 28, 2021, 11:09:30 am
content: There is also a solution for the lack of external power indicator, involving an LED, resistor, maybe some diode and one new hole in the case

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2021, 11:30:05 am
content: <pedant>I just noticed something else:Quote from: Freesurfer on August 28, 2021, 09:22:51 am40C isn't slow, its ultra-ultra-fast. Think supercap bank being charged by an arc welder! Some experimental EV LiPOs have been charged as fast as 10C in the lab,(probably with the fire brigade on standby) but commercial LiPO fast charging seems to top out well under 10C. I said "C/40" not 40C. Remember C stands for the battery capacity (usually in Ampere Hours or milliAmpere Hours), so C/40 means you charge the battery for over 40 hours with a very small current, whereas 40C would mean charging it for little over 90 seconds from empty to full, with enough current to melt stuff near-instantly while trying to keep it cool enough not to explode!</pedant>If you don't 'grok' charging rates expressed as fractions of C, *PLEASE* don't attempt to build your own charger . . .

author: mariush
author_rank: Super Contributor
timestamp: August 28, 2021, 03:58:16 pm
content: It may be easier to just replace your 6 AA batteries with a couple lithium batteries (the pouch style) in series.Basic 2S charger (up to 2S, you can set to 1S or 2S) :https://www.ebay.com/itm/383760467132Another example (up to 3S, you can set to 1S, 2S or 3S) :https://www.ebay.com/itm/123053845631Takes in up to 18v and charges the two batteries in series (8.4v) - it doesn't have balancing but it's not that critical.Two lithium batteries in series will output 6v...8.4v, which is within the range.

author: ajb
author_rank: Super Contributor
timestamp: August 29, 2021, 12:15:00 am
content: How about an outboard solution? Should have about 3x the runtime of AAs, and depending on the keyboard could probably be stuck to the back with velcro:https://www.amazon.com/gp/product/B00ME3ZH7C/

author: Kjelt
author_rank: Super Contributor
timestamp: August 29, 2021, 08:10:06 am
content: Quote from: mariush on August 28, 2021, 03:58:16 pmThat will not solve TS issue because although the charging is now taken care off, the discharging is still unmonitored, running the cells flat.And we all know that LiIons are unforgiving for fully discharging them.So if there is then a need for a BMS circuit that will also monitor the discharge current , cell voltage and cut them off in case of too low discharge.

author: tooki
author_rank: Super Contributor
timestamp: August 29, 2021, 03:11:58 pm
content: Why not just get a 12V power bank and plug that in when you need battery power?