Connecting (And Using) High-Capacity Batteries In Parallel

For those willing to put some elbow grease into it, there is an almost unlimited supply of 18650 lithium ion batteries around for cheap (or free) just waiting to be put into a battery pack of some sort. Old laptop and power tool batteries are prime sources, as these often fail because of one bad cell while the others are still perfectly usable. [limpkin] built a few of these battery packs and now that he’s built a few, he’s back with a new project that allows him to use four custom packs simultaneously.

The problem with using different battery packs in parallel is that unless the batteries are charged to similar voltages, they could generate a very high and potentially dangerous amount of current when connected in parallel. This circuit board, powered by a small ATtiny microcontroller, has four XT60 connectors for batteries and a fifth for output. It then watches for current draw from each of the batteries and, using a set of solid-state relays, makes sure that no dangerous over-current conditions occur if the batteries are connected with mismatched voltages. The code for the microcontroller is available on this GitHub page as well.

An array of batteries with a balancing system like this has a number of uses, from ebikes to off-grid power solutions, and of course if you build your own packs you’ll also want to build a cell balancer of some sort as well. Batteries go outside the realm of theory and into that of chemistry, so we’ll also provide a general warning about working in potentially dangerous situations without specialized knowledge, but you can see how [limpkin] built his original packs here if you want to take a look at one person’s strategy for repurposing old cells.

13 thoughts on “Connecting (And Using) High-Capacity Batteries In Parallel

  1. I don’t get this project. If the voltages are all over, then the device disconnects the other batteries leaving the only one battery connected? So essentially situation is the same as if no extra batteries would have been connected?

    How about not connecting batteries parallel and if you do it, charge those full and then connect?

    Also I don’t see any balancing leads. Those need to be parallel too to make this thing safe.

    1. The writeup is wrong, read the linked page – all this does is monitors the current on each pack, and switches the load if a pack fails. Each pack is connected in series with a fuse and current sensor, then the packs are tied directly together in parallel with no switching at all.

      1. Exactly that is the problem with the system. The over achieving battery takes all the load and the total system capacity is basically unknown at any given time.

        1. This is true in general. Absent a ‘convenient’ saggy chemistry, the only way to know the total system capacity is with Coulomb counters on every cell. True in series OR parallel…

          And, as long as your load isn’t exceeding any single battery’s safe discharge current, there’s no problem for the load to be supported by only the “over achieving” battery. It’ll lose charge until it’s no longer the over achiever, then some other battery will bear the load for a while.

          This is exactly why Coulomb counters are needed. Even if you build a pack from matching batteries, they’re not ACTUALLY guaranteed to age identically.

  2. I dont get it, I may be wrong, and please correct me if I am, a person can always learn something… For me, I have put a few batteries in parallel and I just use diodes. I have designed a few commercial trackers and I use CR123A batteries in with a diode in parallel. I have also done this in parallel with a 3.3v reg from the vehicles 12v source… so basically 3 power sources tied to one point. My assumtion is power will be delivered by what ever has the most voltage

  3. I do not get how this board helps on connecting batteries in parallel, the bigger issue with Lithium batteries in parallel is that if two have very different voltages the one with the higher voltage will try to recharge the lower one with all the current it can provided, normally maximum discharge current is tens or even hundred times higher as maximum charge current, so the battery being charged will blow up, or in this case the fuses will blow up if correctly size.

    When I read the description I assume that there will be one switch on each battery avoiding current between batteries and letting all the magic smoke escape.

    With one switch per battery, another strategy could be taking current from the batteries with higher voltage and switch on more batteries when voltages are the equalize. This means reading current and voltage of all batteries.

    1. Even with multiple batteries at same start voltage if capacity is not the same, the article talks about reusing batteries, the issue will be the same some batteries will discharge faster than others and current between batteries will happen.

  4. QUESTION: If each battery pack being connected in parallel has its own BMS, won’t that device “protect” that pack if another in the parallel group tries to charge it with a current that too high?

    1. It could protect the pack but that is essentially removing it from doing useful work in the system. Not very practical having the bank of packs changing characteristics like that. It has always been an issue in large UPS systems where one weak string can loaf along while the stronger cells get killed with load eventually degrading the system to the level of the weakest string.

  5. Yeah, the biggest problem with recycling these cells is that they will not have the same remaining life and packs of them will not balance correctly. Without the right bms these packs would be pretty hazardous. As far as the series parallel discussion. Paralleling bateries makes more current available, series makes more voltage available.

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