DIY Battery Questions

Equipment Radios & Power
1

I built a 4x3.7V 26650 LFP cell battery pack with a CF-4S30A-A BMS based on OH8STN’s videos. A few questions:

  1. I had charged the new 26650 batteries on a LFP slot charger that came with some 18650 batteries before the build and measure -15.6V for the DIY battery pack on my VOM. Is that likley overcharge from the slot charger?

  2. My VOM measures -15.6V - the Black test lead on the -P pad and Red lead on the +P pad of the CF-4S30A-A BMS. I guess I don’t understand why that’s reading a -V rather than +V (I’m a biologist).

  3. How best to discharge from 15.6V down to a safe operating range? I’m not sure what to use for a load. Can I splice some Power Poles onto a coax jumper and discharge into a 50ohm Dummy Load?

  4. I have Power Poles on wires soldered to the -P/+P pads - can I use my Bioenno battery charger to charge my DIP battery pack? The charger specs are - Output: 14.6VDC, 2A (CC/CV Charge Profile for LiFePO4 Batteries).

I have a Genasun GV-5-mod charge controller on order for use with a solar panel but that’s a few weeks out on delivery.

Thanks! Bret/N4SRN

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I use a DC to DC converter (3 or 5 A max) . Very small, cheap and may be regulated to any voltage under the input line voltage (I use it at 12,5V for SW-3B radio and for uSDX

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I think the BMS will maintain future charges to 14.8V maximum, so I’m just looking for a safe way to drain the battery from 15.6V down to below 14.8V. I have a 12V regulator on my QDX as you describe. I see the (tr)uSDX spec is up to 16V but others have suggested higher duty cycles might blow the finals.

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4

Hmmm… LFP is Lithium Iron Phosphate or LiFePO. After fully charging, allow the cells to “rest” and the voltage for the pack should be 13.6V +/- so 15.6V sounds very high.

My charger for 4x LFP cells stops charging at 14.4V.

Something needs checking carefully in the setup I think.

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5

Continuing the discussion from DIY Battery Questions:

  1. How best to discharge from 15.6V down to a safe operating range?

I am using a voltage drop with a diode instead of discharging.
A single 10D1 diode drops about 1V. Two 10D1s are switched between through, one, and two in series.

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That’s a nice battery! But I’m just looking to discharge from 15.6V to 14V once, as the BMS will limit charge thereafter. I think the cheap slot charger I used for the initial charge has poor overcharge limiting.

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7

Maybe check your VOM on a known battery, eg the one in your automobile. That will certainly be connected the right way round (+ and - as marked) and should read somewhere between 12 and 13.5 volts or so with the engine off. Make sure the black and red leads are not reversed at the VOM…
If that reads correctly, something in your project is not right - check it all carefully, is the BMS the right way round? (I don’t know if it is possible to connect it backwards without blowing it up, but the polarity marked on the batteries will be correct. The polarity on the BMS will depend on your wiring…).

You could do that, a 50 Ohm load will dissipate about 5Watts at 16V. As long as your load is rated for at least 5W, it will be fine.

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8

When I test my batteries to check the present Amp hour I just discharge them with an automotive lamp. I personally happen to use a 1157 double filament bulb for my 3 cell 12.6V packs. Just solder a couple wires to the bulb. For me the “brake light” filament draws about 1.5 amps and the “parking light” filament draws about 350mA. Depending how fast I want to discharge the pack I connect one or the other or sometimes both just to speed it up a bit for a short time. Cheap and easy…works for me.

Tim - K5DEZ

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9

Hi Bret,.

Well it is important to make sure you have a reliable measuring instrument. My old VOM had a polarity reversal switch so that if it could allow a forward reading regardless of whether a positive or negative polarity was connected to the red lead .

Set the switch to normal polarity, plus on red common on black.

Now your LFB, otherwise referred to as LiFe battery should read a positive voltage when you connect red to positive, black to negative. If not maybe you have wired the battery the wrong way? Or has someone been modifying the meter?

You must get this right or else you will destroy your equipment because you can’t measure polarity.

However, I don’t think you have LiFePo cells. The nominal voltage you quote of 3.7 V screams LiPo cells which are fully charged at 4.2 V per cell or 16.8 V for your battery.

Now 15.6 volts could be either a reading or recording error on your behalf or the VOM. That would be easy to fix. Let’s suppose both are right.

Now the bad news. 15.6 V is a good volt less than the battery should be charged to. Under 80 % charge. Your BMS might be responsible

It won’t hurt the cells, you just lose some capacity as they are not charged to 90 % or a bit
more.

Given the charger was for 18650 cells it is most probably intended for LiPo cells that should be charged to 16.8 V for your battery or 4.2 V per cell.

I have not seen LiFe cells that are in a 18630 package or a 26650 package. If they are indeed LiFe cells then you have damaged them by charging to 15.6 V. They may still give a useful life if you don’t do this again. But I think the 3.7 V is a give away and they are LiPo cells and undamaged

Over charge and over discharge can both wreck perfectly good cells.

I strongly suggest you read manufacturers data on charge currents and voltages. ASAP.

I have a regulated power supply that is set to 21.0 volts and 600 mA for charging a five pack of LiPo cells for my old electric drill. It keeps the charge current below 1/3 C and there is no over voltage.

I use other settings for other batteries.

Unlike a smart charger, I do not monitor the cell temperatures, which I should.

73
Ron
VK3AFW

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10

These are widely available in both these sizes. (I assume you meant 18650).

Just to be picky rigid cells in 18650, 26650 etc cylinders are not LiPo cells. They are usually LiIon but can also be LiFe. LiPo cells are in pouches but could have a hard outer case.

[Of course all LiPo, LiFe and LiIon cells are lithium ion but LiIon usually refers to the rigid cells using 3.6/3.7V chemistry.]

Having said that, the OP cannot have 3.7V LFP cells. So he either has LiIon cells or has badly overcharged them.

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Hi Richard,

Yes that is a typo, should have been 18650. Sorry

Interesting about the alternative chemistry, Several searches on the Internet here have failed to come up with anything but LiPo cells in those two sizes.

The round LiFePo cells finish up in replacements for Pb gel batteries in the same old cases but I don’t see them individually. Must be a northern hemisphere thing?

73
Ron
VK3AFW

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12

They do seem to be less easy to find Down Under. For example, RS stock them here but not in Australia.

I did find 18650 LiFePO4 Battery 1600mAh 3.2V Rechargeable | Wiltronics and 26650 LiFePO4 Battery Cell 3000mAh 3.2V | Wiltronics.

13

Ah ha,
Thanks. Worth knowing they exist. Not a company I am familiar with. Not cheap either.

73
Ron
VK3AFW

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14

Also stated further up the thread…verify your multimeter is measuring correctly.
Tim - K5DEZ

15

My bad - I connected the BMS to the cells in reverse polarity. The +/- symbols were very tiny! The cells were in deed Li-Ion. I’ll have some reworks to do! Thanks for the help, all.

Bret/N4SRN

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Glad to hear that is sorted Bret. I must learn to differentiate between LiPo and. Li Ion cells when talking about them.

I either use LiFe cells or a switch mode regulator with other lithium cells.

73
Ron
VK3AFW

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17

I keep seeing all these different questions about lithium cells, and reading about all the fires they are causing.

If you’re unsure about how to hook them up and use them, buy pre-built systems from people you can sue if you’re still alive after they fail. Buying cheap stuff from Asia and hooking it up backwards seems to be asking for problems.

I’m sticking with my nimh and gel cells!

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18

Rick,
Doesn’t matter how much you pay, hooking stuff up incorrectly is going to cause problems. Even the antiquated gel cells can let smoke out if reverse wired.

For your next battery do yourself a favour, buy a LiFe battery. So much lighter than the gel cells, more stable output voltage and just as safe.

Even with the Li Ion or LiPo cells it is abuse that leads to fires and even then the percentage of times it happens is very low. Over charging, high charge rates, short circuiting, all bad for any battery.

Overhyped reporting has blamed batteries for problems caused by people.

73
Ron
VK3AFW

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19

[Of course all LiPo, LiFe and LiIon cells are lithium ion but LiIon usually refers to the rigid cells using 3.6/3.7V chemistry.]

Having said that, the OP cannot have 3.7V LFP cells. So he either has LiIon cells or has badly overcharged them.

Ron

So here’s 3 different lithium chemistries, each with it’s own charge and discharge requirements, and I’m still wondering which one the OP is using. Not sure he knows!
Then charge one way, discharge to get to required voltage while things are backwards.

Im not saying lithium is bad. Im saying if you dont know what you’re doing, buy pre-built SYSTEMS that eliminate the quesswork.

Im not staying with gel cells and nimh because they’re better. It’s because I know them!

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Yes, the batteries are Li-Ion but have 3.7V printed on the jacket.

Bret/N4SRN