Off the shelf charger for a 9.6V Nimh pack or knock something up? - Page 1
Forum section: electronics / projects

author: TheBay
author_rank: Unknown
timestamp: March 04, 2022, 12:18:14 am
content: I've got a Kenwood TH-D7 V2.0 G handheld transceiver that was designed in the late 1990's. (Full Duplex for satellites and a TNC)I want to keep this running for as long as I possibly can as there is no modern day equivalent and I love this radio.The problem is it was designed to use Ni-Cad batteries, which are close to impossible to obtain, especially the 9.6V pack which allows it to operate at full transmit power.I found out that there is a 9.6V 1100mAh battery available that is manufactured by a third party but there is very little information out there about it, I have scoured the internet and come up with very little. This initially looked very interesting to me as in the pictures I could see a coaxial DC input on the back off the battery covered by a rubber bung. I thought what a great idea someone has put some circuitry inside this pack to simplify charging.I decided to order one and it took around a week to get here from Europe.But looking at this battery initially I noticed that the DC input socket on the back has it's polarity inverted, i.e. centre negative which I thought was strange as it cannot work with the 13.5V DC adapter that comes with the radio, even though it is exactly the same size coaxial dc jack.I checked it over with a multimeter and indeed it has its polarity reversed, so someone at some point had designed this with a specific charger/power supply in mind.Further checks were carried out and it appears that the DC jack is just connected to the battery with a diode in series and I do not think there is any logic inside at all.I spoke to the seller who didn't sell the charger but said it will be fine to charge through the radio, however I'm reluctant to do this for 2 reasons, firstly the radio was designed to charge Ni-Cad batteries and has very very basic charging circuitry, looking at the schematic for the radio it is just a basic constant current of 70mA with a voltage of around 13.5V. This will not only be extremely slow it is not designed for a Nimh battery and I'm concerned about the longevity.I put it on one of the bench PSU's today and thought I'd charge it up that way, I set the PSU to about 12.55V and a CC of 200mA, I had every intention of setting it to a rate of C/10 (C0.1) but was busy with other things and wrongly assumed it had a capacity of 2000mA not 1100mA.The battery was low when I put it on as it had been used on the radio today. I set the PSU to 12.55V due to the series diode so the max voltage at the battery would only climb up to 12V.I monitored it in the workshop by watching the voltage/current on the PSU, also this battery has a NTC so measured the resistance on those terminals to keep an eye on the cell temperatures. It was probably on the PSU around 5-6 hours. I had to take the dog out so didn't really want to leave it on charge so terminated it when I could see around 11.98V on the battery (Not the PSU reading due to the diode voltage drop)It was then I realised that I had it set to 200mA not 110mA as intendedI do hope that hasn't caused any issues? The PSU was still showing a CC of 200mA and the battery did not feel warm at all, the NTC reading had only changed around 1K from the reading I took at ambient.Now I know CC isn't an ideal way of recharging a Nimh pack and using one of the bench PSU's is quite inconvenient, I do not want to charge it using the radio as it will take forever and I am concerned about the longevity of this and want to keep it going as long as possible.So I'm at loss on what to do to charge this, it would be great to use the coaxial DC input jack. I was looking for a Nimh charger with logic but couldn't find anything and also there is the series diode voltage drop.Or is there something I could knock up on some veroboard that would work, looking at various datasheets and guidelines on how to properly charge a Nimh battery pack this looks quite involved.Excuse my long post, I thought I would post here as someone here has far more experience with this battery technology than I have and have come across a similar situation to this.P.S. anxiety is running wild, have I prematurely damaged this battery with a charge rate of C0.2?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 04, 2022, 12:55:56 am
content: Adapt this design if you want to build your own:http://www.ultrasmartcharger.com/Or just get a R/C universal charger that supports NiMH. That would be particularly good if the battery is old and only have a few years or less of life left, at which point you can swap it for a lithium pack with BMS, then keep using the same charger.

author: TheBay
author_rank: Unknown
timestamp: March 04, 2022, 10:56:01 am
content: Quote from: NiHaoMike on March 04, 2022, 12:55:56 amThanks for the link, looks a good project but only charges 2 cells?I do have a ISDT Q8 that I use with Li-ion, Lipo & LiFePO4 batteries and it can do Nimh but I have no idea if it has any logic behind the charging or just just a simple CC/CV source when in that mode. Also it is a pain to use just to charge this particular battery and not that portable having to take a PSU with it and leads when I go away with the radio.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 04, 2022, 12:40:03 pm
content: Quote from: TheBay on March 04, 2022, 10:56:01 amIt can be adapted for more, just the matter of using a voltage divider to divide down the voltage and replacing the current source with one that operates at a higher voltage.