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Homemade Lithium ion battery bank questions

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Cortland

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Hello all, I'm new here. I'm seeking knowledge and advice about a Lithium ion battery bank that I made. I got these batteries from work out of a device that we replaced the batteries on. It is a somewhat proprietary battery as it is made specifically for this device and not open to the general market and I have limited knowledge of the specs outside of voltage and mA hours.

It is made up of 10 batteries at 7.4V each and 1900mA hours a piece.

5 batteries are in parallel and each bank of 5 is wired in series making a battery bank of 9.5 Amp hours and 14.8 Volts. They are already soldered up and are sitting at about 16V's.

Now a little bit more about the batteries, each battery pack is actually made up of 2 smaller batteries in series. Attached to this pack is a small circuit board which I believe is some kind of protection circuit but I'm not really sure. There is also 3 wires coming off of this battery. A black a red and a yellow. Red is positive but it also seems to be the common. Red and black reads about 8.3V. Red and yellow reads about 7.6. And black and yellow reads 0V. I wired up this battery using red and black and ignoring yellow as it was a smaller voltage and I didn't know what it did.

The "stock" charger for one of these batteries is 10VDC at like 1.8 amps if I remember correctly.

I'm wanting to use this battery for a fish finder or whatever I find use for it. I have a 2 year degree in biomedical electronics but it's only enough knowledge to get me into trouble haha. My concerns are safety and here are some of the questions below.

1) What do you think the circuit board attached to the battery is?

2) What do you think the yellow wire is used for? Some sort of feedback? Just another power source for something on the device to run off of?

3) How can I charge this bank? I don't even need this battery to be at full voltage as the depth finder is a 12V device (max voltage is 17V). Will hooking up my 12VDC 500mA charger be sufficient enough to keep this thing charged and not blow up on me? I'm not looking for ideal here just enough to keep this thing going.

4) Does the circuit board on the batteries require feedback to work properly?

5) Should I get a 14.8V smart charger or do I just need a 7.4V smart charger? Or should I take one of the power supplies that is made for these batteries and hook it up?


That is all I can think of for now. The battery is stable but I'm not going to hook it up to anything or try and charge it until I know it's safe.
 

dknguyen

Well-Known Member
Most Helpful Member
What type of device did these batteries come from? Circuit boards on lithium batteries (like laptops) tend to be for safety monitoring and gas-gauge monitoring purposes. And if it's anything like a laptop battery, you won't be able to charge (or even use it sometimes) without cooperation from the PCB.

But 2 cell lithium batteries often have 3 wires anyways even with no PCB. The first one is ground and the other two allow access to each individual cell. This is for balancing purposes so one cell doesn't not become overvolted during charging which is really bad and dangerous for lithium batteries (we're talking burn your house down dangerous, even when you're standing right there to do something about it).

Lithium batteries have a nominal voltage of 3.6V per cell, 4.2V max.

Is your smart charger made to specifically be able to charge lithium batteries? If so, is it made specifically to be able to charge any battery up to 14.8V or only 14.8V?

Do not just hook up normal power supplies to a battery to charge it. You can't get away with this in lithium batteries. Lithium batteries are vulnerable to both overvoltage and overcurrent on charging. There isn't a line between ideal and non-ideal for charging lithium batteries. There is only safe and dangerous.

Hooking up a 12VDC supply to your 4-cell pack will discharge all the cells down to 3V (if it stays balanced across all cells which it probably won't) damaging them due to undervoltage, as well as overcurrent. Hooking 16V to your 4-cell pack won't under or overvoltage the pack but will still encounter overcurrent issues (either into the battery or out of the battery depending on which voltage happens to be higher). You need a charger made for the job.

Radio control aircraft and truck enthusiasts know all about this and go to great lengths with their chargers. Some go so far as to never charge battery packs in parallel since the fact they can be disconnected from each other means they can be discharged at different rates to different voltages. What I mean is, don't go connecting lithium cells in parallel willy nilly. You have to make sure they have the same voltage before connecting them to prevent excessive currents between the batteries and once they are connected you're okay as long as you always keep them connected in parallel. If you disconnect them from parallel then you have to charge them up to the same voltage and check before connecting them in parallel again.

My advice after figuring out what the PCB actually does, is to look for a lithium charger from a radio control model hobby shop. They charge varying sizes of large lithium batteries all the time. Note that the more powerful chargers don't run straight off wall voltage (so they can be charge a lead-acid battery in the field) and in order to do so require an AC-DC power supply which often costs just as much as the charger.
 
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Cortland

New Member
Okay so I won't just use a regular DC charger. With this battery pack what voltage of lithium ion charger would I need? This is out of an infusion pump. Medical grade batteries.

What would happen if I used the charger that this device would normally run off of (10VDC at 1.8A made for lithium ion)? I took the charger apart and it's just 2 wires coming off it. Would I just have to separate the series connection and charge with parallel strings individually (string of 5)? The parallel strings are the "stock" voltage just the capacity is bigger. So the one charger would be charging 5 batteries rather than its normal 1.

When I hooked these batteries up in this circuit I did not make sure each one had the same voltage and nothing bad happened.

How do I figure out what the function is of the PCB on the battery pack is?

I'm leaning towards just going with my normal lead acid battery and not deal with the danger or the unknowns here. Or never charging this pack and when it's out just throw it away (dispose of properly of course).
 

dknguyen

Well-Known Member
Most Helpful Member
Okay so I won't just use a regular DC charger. With this battery pack what voltage of lithium ion charger would I need?
Any charger labelled to support lithium, lithium-ion, or lithium-polymer cells will work as long as it supports up to your number of cells. Lithium cells are 4.2V max per cell and 3.6V nominally.

When I hooked these batteries up in this circuit I did not make sure each one had the same voltage and nothing bad happened.
It's more of a precaution than anything else since the worst case scenario is really bad (hooking up a full battery in parallel with a half depleted or depleted one might cause a high current discharge enough to heat and ignite the batteries, or at least make them puffy and damaged so they are no longer safe for use.

What would happen if I used the charger that this device would normally run off of (10VDC at 1.8A made for lithium ion)? I took the charger apart and it's just 2 wires coming off it. Would I just have to separate the series connection and charge with parallel strings individually (string of 5)? The parallel strings are the "stock" voltage just the capacity is bigger. So the one charger would be charging 5 batteries rather than its normal 1.
This should be fine. It shouldn't have any issue charging batteries in parallel but it will take much longer to fully charge, obviously. Some people do not like charging packs in parallel though but if it's already connected and stays connected then it's already balanced and will stay that way.

What do you mean by separate the series connection? Did you meant to say parallel?

How do I figure out what the function is of the PCB on the battery pack is?
That's tough tbh. Could you show us a photo of as much of the PCB as possible?

I'm leaning towards just going with my normal lead acid battery and not deal with the danger or the unknowns here. Or never charging this pack and when it's out just throw it away (dispose of properly of course).
For a stationary setup where mobility is not a concern, I really would recommend going with lead-acid. Safer and takes more abuse and low energy density and weight aren't really concerns there.

For application where you leave the batteries charging unattended, do not use lithium. RC modeller guidelines are to never charge batteries unattended.
 

large_ghostman

Well-Known Member
Most Helpful Member
Why did they get rid? If its a service thing then fine, if its because of a fail then stay away. I would think its likely a standard switch over every 12 months. Make sure you use a decent charger those batteries are not used to abuse. I use an infusion pump, its really funny about the batteries it takes, if i try a different one the pump destroys it.
 
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