DIY LiFePO4, the build begins

Four 280 amp hour LiFePO4 cells ready to become a 12-volt battery

In the last couple of months I’ve installed two of the three main types of lithium iron phosphate (LiFePO4) batteries. In March I installed Mastervolt’s system integrated MLi batteries on Have Another Day and just last week I finished up the installation of Battle Born’s 8D drop-in batteries on another boat. With two out of three types covered I figured it was time to get my hands dirty with the third type, a do-it-yourself build of a 12-volt LiFePO4 battery.

For my DIY LiFePO4 battery, I picked four, 3.2-volt, 280-amp hour cells direct from China via Aliexpress. The batteries took about two months to arrive, which isn’t surprising for heavy items with the current international shipping challenges. The seller says these are brand new cells and they look it, though I’m not sure I’d know if they weren’t. I selected an Overkill Solar 4S, 12-volt, 120-amp BMS paired with my batteries. Although the BMS was back-ordered, it arrived a few weeks later and well ahead of the batteries.

The batteries’ specs aren’t overly clear but they appear to be able to sustain 1C continual load or 280 amps. I don’t have a specific application for this battery yet and felt that 120 amps was likely to be enough for any use I dream up for the batteries, so I stuck with the 120-amp BMS.



Top balancing underway with all four cells wired in parallel

The first step of my build is to get all four batteries balanced at the same voltage. In the picture above all four batteries are connected in parallel and a 3.65-volt charge is being applied. As you can see on the multi-meter, the batteries are about 0.3 volts below that charge so it will take a while for them to reach top balance with all four fully charged and balanced with each other. Once that’s done it will be time to move on to connecting the BMS and physically connecting the four cells into a single battery.

I’m a rookie, so I’m taking it slow and being as careful as I can. If any of you who have done this before have words of wisdom to share, I’m all ears. Otherwise, I’ll just be here waiting for my meter to edge up to 3.65 volts.

Ben Stein

Ben Stein

Publisher of Panbo.com, passionate marine electronics enthusiast, 100-ton USCG master.

13 Responses

  1. Ben Stein Ben Stein says:

    Top balancing takes a long time. I have a 10-amp bench power supply and I’m trying to top balance 1,120 amps of 3.2-volt cells. In about four hours I’ve brought them up .0012 volts, they need to come up about .35 volts.

    -Ben S.

  2. 2,5 A per battery, sure, that is going to take a long time. Lithium cells have a pronounced hockey-stick charge – voltage diagram, so you will find that they remain stable in voltage until they are _almost_ full then the voltage starts rising quickly.

    > Iโ€™m trying to top balance 1,120 amps of 3.2-volt cells

    Ben, please don’t be casual with units when batteries and electricity are involved. It is hard enough for readers to keep their heads around this without you writing “amps” when you mean “amp-hours”. You meant to write “1,120 Ah of 3.2 V cells”. “Yes but it is hard” is IMO no excuse when you are a journalist writing about this.

    • Ben Stein Ben Stein says:

      Kees,

      Mea culpa, I did indeed mislabel the unit of capacity on the batteries. Wired in parallel, the four 3.2-volt, 280 amp-hour cells yield a total capacity of 1,120 amp-hours.

      Also, indeed as you mention, given the very stable voltage of LiFePO4 chemistry batteries I don’t expect much fluctuation in voltage until the cells get very close to 100 % state of charge. I’m at about 18 hours of charging right now, which means I’ve probably replaced about 180 amp-hours of energy, perhaps another day or two to go.

      -Ben S.

      • Joel says:

        But as a 12 volt system it will be 280 aH.

        Joel

        • Ben Stein Ben Stein says:

          Yes, you’re correct. The 1,120 Ah figure is only relevant while top-balancing the cells in parallel. So, once this step is done I can charge at 12v nominal (higher actual voltage from the charger) which will cut charge times by four.

          -Ben S.

  3. Joel says:

    Ben, I was thinking of buying a kit similar to yours here: http://lithiumbatterykit.com/.

    Is there any way to judge the quality of the cells?

    • Ben Stein Ben Stein says:

      I don’t know of a way to assess the quality of the cells. I think you could contact the seller and ask if they have any reports on the cells. But, overall, you’re likely placing your trust in the reputation of the seller. In this case, I know nothing about the seller, so you may have a leap of faith involved.

      You’re definitely paying for the top balancing, US warehousing, and kitting the seller has done. My total cost for the 280 Ah batteries and BMS was around $600. There are additional components included from this seller that might add another $100 or so. But, I also have all the tools, wire, and connectors needed on hand which makes the build easier for me.

      The one question I’m left with is how the seller can ship these batteries top balanced. My procedure for top balancing will have me connecting to the BMS fully charged. My understanding of U.S. shipping regulations limit batteries to 30% SOC for shipping.

      -Ben S.

  4. Brian Stannard says:

    I wouldn’t purchase cells that were top balanced by the supplier. LiFePo4 should not sit fully charged. If the supplier top balances how long do they sit on a shelf fully charged – months?

    As far as supplier I purchased Calb cells from one of the main distributors in China. Much less expensive that buying from elsewhere.

  5. Andreas says:

    I had in mind to do exactly what you are doing but I’m stuck on something. How do I charge these with an alternator? When they are fully charged, the BMS will disconnect them and the resulting spike will damage the alternator. Sterling makes an alternator protection device. That might work. I’d prefer a BMS that could signal an impending disconnect…

    • Ben Stein Ben Stein says:

      Andreas,

      First, at least with regards to the BMS I’m using (from Overkill Solar), the BMS doesn’t disconnect when the battery reaches full charge. The cells taper to accepting nearly zero amperage, but there’s not a disconnect event. The only time there’s a disconnect event is when a cell or the battery as a whole violates the over-voltage thresholds. In my normal charging thus far, with the bench power supply set to 14.4v I haven’t seen this event. I did force it by setting the power supply to 14.8v with a fully charged battery. That quickly resulted in the BMS shutting down charging — as it should.

      I think (but I’m still learning here as well so everyone should feel free to chime in) you have several options to avoid damaging your alternator. First, and possibly the safest option because it changes the charging system as little as possible, would be to maintain a lead-acid battery for engine start and then use a DC to DC converter from the lead-acid battery to the LiFePO4. That way your alternator is only connected to a battery that’s not ever going to disconnect. The second option is indeed an alternator saver like you mentioned. I don’t believe the Overkill BMS has an external trigger to shut down the alternator, though it would be nice.

      -Ben S.

  6. Andreas says:

    I was hoping to build four 280AHr battery packs, and wire them in parallel.. that would give me 1120 Ahrs. That’s 13.44kWh ๐Ÿ™‚ I could even run a small A/C unit off my inverter for a few hours ๐Ÿ™‚
    To recharge that battery bank, I actually considered not using the alternator at all, and just going with shore power/genset and solar charging only. Not as nice as having the alternator charge while we cruise, but very usable. I’ll figure it all out eventually.. I’m also still learning about having several LIFEPO4 batteries in parallel.. each one has 4 cells and a BMS.. what happens when one of the prallel packs experiences a problem and disconnects? I almost need a BMS to keep track of the battery pack.
    Maybe I’m overthinking this.. So much to learn ๐Ÿ™‚

  7. Val Vechnyak says:

    Andreas,

    Good question you brought up in your first post. This great article talks about two different alternator damage situations and ways to protect them.

    https://livsailing.com/2019/11/04/protecting-your-alternator-when-switching-to-lithium/

    Still, personally, I would resort to only DC to DC charger simply because I can never be comfortable pushing 80, 120, etc. amps that big alternators put out through the wires on my sailboat.
    My needs are pretty conservative. Even with all possible systems turned on i only use 5-6amp hour. My battery is 200ah.
    With the big battery bank like yours my solution may not work for you.

  8. Richard R says:

    Will Prowse tested these batteries and was impressed.

    https://www.youtube.com/watch?v=3U4ZfQ_IToI

Join the conversation

Your email address will not be published.