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I'm looking into building my own LiFePO₄ pack and I have my eye on a 50 Ah, 3.2 V cell that goes for about €50. I need 36 V, so that's 12S or €600.

I want to be able to charge it fast. On the 3.2 V cell datasheet it says that it likes to charge at 0.5 C. I'm trying to find 20 to 25 A, 36 V (43 V) LiFePO₄ chargers and I'm having a very hard time finding any at all. There are none on the "reputable" source's website from which I'm planning on getting the cells, but looking at other (48 V, 60 V) chargers rated at 1000 W, they go for about €250+. The price of the battery is already quite a hit to my budget (yes, I am poor). I really was not prepared to spend half that on a charger (if I had found one at all), so naturally I started thinking how I can go around this.

I can easily get used PC PSUs from my job for free, which are rated at 500 W. Their 12 V rails can typically supply about 30 A, so if I wire two of them in series I'd get a 24 V, 30 A, 720 W supply. Then I could get something like the following for like $30:

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Which claims 30 A input, which is perfect. I'd probably need to add some more fans but that's okay. I'm guessing output will be around 15 A at 43 V so the battery would need more than 2 hours to charge, which is far from ideal but for this price I'd say is more than acceptable.

If I set the voltage to 3.65×12=43.8 V which is said to be the maximum cell voltage, would that take care of the overvoltage problem? I mean how can cells get overcharged if the charging voltage is less than or equal to their maximum voltage?

Obviously, I have no idea what I'm doing and I'm still at the research phase, so I'm looking for advice. Is this even going to work or am I going to set my shop on fire? I know you will be tempted to tell me to get a proper charger and believe me I'd prefer that too, but it is just way out of my pocket.

php_nub_qq
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    Computer PSUs are typically grounded - 0V is connected to mains ground, so connecting two in series will short-circuit the lower voltage one. This could (depending on the PSU) possibly be avoided by cutting one of the ground wires or using an isolation transformer, but I can't tell you whether it's safe. – user253751 Jan 07 '23 at 20:18
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    @user253751 I can tell you it is not safe. The ground is not supposed to be bypassed, and the supplies are not supposed to be connected in series. That does not prevent people doing so, it's just that is not the purpose of computer supplies and if something goes wrong it may blow up both power supplies (and maybe external boost converter and batteries). – Justme Jan 07 '23 at 20:52

3 Answers3

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No, you can't. A battery requires a charger (limited current, precisely set top voltage) not a power supply (not current limited, non-adjustable voltage).

If you do connect a power supply to a Li-ion battery, the following may happen:

  • The cells are damaged
  • The power supply may overheat or be damaged
  • The BMS may shut down

You can use that "PC PSU" if you also provide a current-limited, adjustable voltage, DC-DC converter between the power supply and the battery's BMS.

The item you linked to is an LED supply, which does provide limited current, but does not have adjustable constant voltage stage.

Davide Andrea
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  • Oh boy. Well [it](https://www.ebay.com/itm/334672088165?hash=item4dec02cc65:g:5-YAAOSwggtiVkEd&amdata=enc%3AAQAHAAAA4DbffNC2YQ5JXhqcWNSZv9EPJHzeEUec58ROk%2Bw3xt1yoRPEnAvnROVX5ejY9gZfmQc5kXuiumObO%2BhMP0Soz%2BwEMRAm9lsEXzu1sOHieWzX1yEa8JAGBdVU%2B7cJqVBaYoWOPCvs5l4ko9Ndt1wa5Dqe1Op4ORh%2FRyYHX%2BNX59RmZA%2F%2BXfF74gVFwrAf3neeTuqshr9zVqCuyqQ0LjURBHWDLduXL9qprNLm38Lpu1Te%2Fqu6j%2BA0cEW%2F7PNyhhGr9NvWGrNklmelKOyMK207sTwIbAAwOswzQ0P2vuOxD6r6%7Ctkp%3ABFBMyPig2rFh) has 2 pots, so I'm guessing it also has a constant voltage mode but that would require monitoring during charging which would be a bargain. – php_nub_qq Jan 07 '23 at 20:40
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The product is not a charger, so it can't be safely used to charge any batteries.

Even if it has constant current limit and constant voltage limit, it still lacks the detection of when batteries are full and when to stop charging batteries.

Justme
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  • Isn't that a job of the BMS? – php_nub_qq Jan 07 '23 at 20:56
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    No. It definitely is not the job of BMS. BMS will protect from overvoltage and overcharge, for example if the charger fails to terminate charging. – Justme Jan 07 '23 at 21:04
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    So if my not-a-charger doesn't stop charging, then the BMS will? – php_nub_qq Jan 07 '23 at 21:07
  • Yes, eventually, with the batteries slightly degrading from slight overcharge each time you charge them. That is not how batteries should be charged. – Justme Jan 07 '23 at 21:10
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    What if I charge at a lower-than-maximum voltage? For example if my battery is 12S then it's supposed maximum voltage would be 43.8V, what if I charge with 41V, and have my maximum SoC be around 95%, would that take care of the overcharging problem? Obviously I'd have to make sure my battery is not already above 41V before starting to charge, but if I only use this charger then it would be impossible to go over 41V anyway. – php_nub_qq Jan 07 '23 at 21:12
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    I am not expert at lithium battery charging, but what I recall they should never be float or trickle charged. The charging should be stopped. You can easily search the web on opinions on this. But it depends on how you want to use your batteries. Either you misuse the expensive batteries and accept the risk they degrade and damage, and may become dangerous, may explode or burn down your house. Or you use them properly with less risk of them getting degraded, damaged and exploding and burning down your house. It all boils down if you want to cheap out and take risks. – Justme Jan 07 '23 at 21:36
  • @Justme that applies to lithium-ion and lithium-polymer types, but from what I've heard (and I am no expert either, so the asker should do their research) LiFePO4 (lithium-iron-phosphate) batteries are a lot more resilient and do not explode as easily. However, they are still damaged by overcharging – user253751 Feb 22 '23 at 00:02
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A charging circuit for series Lithium cells is "balanced" so that it detects the voltage of each cell and adjusts their charging currents and voltages so that the weakest cell that becomes fully charged first does not become overcharged when the rest of the cells are still charging. Then the next cell then the next cell etc.

Audioguru
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