Electronics and battery help/advice

IAmATeaf

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I have an old multimeter which can be powered using an external PSU and via 6 C cell batteries. The external PSU I use is 6v but the meter can accept anything from 5v up to 15v. Internally the 6 C cells are in series so 9v in total. When the external PSU is plugged in it effectively disconnects the negative of the internal battery pack via the PSU jack when the PSU is plugged in.

Now what I wanted to do is replace the internal battery pack with some 18650 batteries that I have spare, the plan was to use 4 batteries, 2 in series then the pair paralleled together. So I’ve ordered a 2S BMS board and the required better holders.

The BMS has connections for B+ and B- and BM which will go to monitor the individual cells. This is where I’m getting a little confused, the BMS only has P+ and P- which from what I’ve read serves as the charging supply and the output. The minimum charging voltage is 8.4v so if I understand it right the BMS must detect voltage over 8.4v and flick it into charging mode?

My plan was to replace the existing 6v PSU with a 9v PSU which could then be used to both externally power the meter and to also charge the batteries.

This is where I’m getting confused, with the existing power jack on the meter when the external PSU is plugged in it disconnects the negative of the internal battery so how would/could I wire it so that I can use the external PSU to power the meter and to charge the new batteries.

Could safely rewire to not disconnect the internal battery negative so that way both the meter and BMS are connected when the PSU is plugged in or must I disconnect the load when charging?

Any help would be appreciated.
 
The minimum charging voltage is 8.4v so if I understand it right the BMS must detect voltage over 8.4v and flick it into charging mode?
It will start charging if it is below maximum charging voltage, which is OK, although you probably don't want to leave it plugged in to charge long term.

Could safely rewire to not disconnect the internal battery negative so that way both the meter and BMS are connected when the PSU is plugged in or must I disconnect the load when charging?
The disconnect is to protect the non-rechargeable cells from being charged.

You must not connect the input to your BMS to the output of the BMS, otherwise it can't work. You must disconnect the device from the PSU and only power it from the battery output, this is OK.

Have you got a low voltage cutoff? Maybe you are using "protected" cells, in which case you are OK.

You don't appear to have a charger, which is an issue, the charging voltage must never go higher than the max voltage of the batteries, and there should also be a current limit. If you charge higher than max voltage then the BMS will disconnect and you will not get any charging done.

Be careful with lithium batteries! 18650 cells can explode if they are poor quality ones and the vents don't work.
 
I can understand using what you've got and ditching the 6 C cells, but LiIon doesn't like being left idle with more than a 50% charge. I'd recommend another battery technology instead, like AA-sized NiMh Eneloops in adapter sleeves to fit the C cell holders. In this use they will perform similarly to what regular C cells would as far as use-time before charging.

If you still want so use the 18650's you'll need to either disconnect the load while charging or make sure the meter is switched off along the power leads so as to not 'confuse' the BMS. Or you can use the pack externally connecting through the PSU port. And of course you need the proper charger for LiIon's- using anything else can be dangerous.

Phil
 
I was under the impression that the BMS would take of charging/discharging the cells as needed, is this not correct?

I was also thinking of leaving the existing PSU socket as is and adding another socket just for charging, would this be a better option?
 
I was under the impression that the BMS would take of charging/discharging the cells as needed, is this not correct?
Normally a BMS is only for protection, if you put a too high voltage in then it will simply turn the charging off to protect the batteries. You supply it from a charger of some sort which should have a current limit and a voltage limit. The sort of charger used for radio control models might do what you want instead of using a 9V PSU, presumably you only need a cheap low current one.

I was also thinking of leaving the existing PSU socket as is and adding another socket just for charging, would this be a better option?
Yes, but make sure you use different types of socket if plugging into the wrong one would be dangerous.
And it probably needs to disconnect the device from the batteries (as the current one does) so that they don't get charged bypassing the BMS.

Phil is right about NiMh Eneloops, that is a simple solution and they work well.
 
I was under the impression that the BMS would take of charging/discharging the cells as needed, is this not correct?

I was also thinking of leaving the existing PSU socket as is and adding another socket just for charging, would this be a better option?
The BMS will handle charging within it's limits as long as only the battery is connected to it- no other loads while charging.

As far as the existing socket, maybe you can just tuck it inside (insulated of course) and put your new socket in the hole it came out of.

Lots of ways to skin a cat as long as you're careful with the LiIons :rolleyes:

Phil
 
It might be easier to charge your 18650 batteries externally in a normal NiMh/LiIon battery charger if they are reasonably easy to extract. It has become normal these days not to do that but it still works fine for torches etc.

Another option is to connect the cells in parallel and charge them from a USB charger using a "LiPo Battery Charger TP4056 Module", that has all the protection you need built in, only issue is that the output voltage is too low, but that can be fixed with a step up voltage regulator, but you will need a switch to power it on/off otherwise it will slowly drain the battery.
 
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I’ve down some more reading, should have maybe done that before [emoji3] I know understand the charging capabilities or lack off of the cheap Chinese BMS so I’ve order a couple of TP4056 charge boards and will use them to perform the charging.

Am still green and learning and doing plenty of reading [emoji3]
 
I’ve order a couple of TP4056
Can I ask why a couple? Wondering if you are intending to use them in series, which I'm not sure is a good idea...
 
I ordered 2, so one for each bank of batteries.
 
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