Tuesday, June 13, 2017

Low Cost Active Battery Balancer

You might have noticed in my last posts that I used 120 pieces of recycled 18650 lithium cells alongside with 4S Lipo pack to power my ebike.

This introduces large variance in terms of practical charge capacity and coulometric efficiency. As the result, charging them all in series configuration (14S pack) can be very challenging because each individual cell accepts charge at a different rate, meaning, all 14S will sit at different voltage levels.

The issue arises when at least one of them reached overvoltage condition. For my case, anything above 4.2V per cell is deemed to be very dangerous as it might explode and catch on fire. The tragedy happened to me once before (fault charger) and I learned the hard way to respect the voltage limit.

I then constructed a passive battery balancer which was TL431-based shunt regulator (refer to my Hackaday link below). It works fine and dandy, but, the shunt transistor overheats pretty much all of the time. Dissipating 300mA as the balancing current at 4.2V (that's 1.26W) proven to be stressful for the design.

Not to mention the fact the balancer needs all cells in the pack to reached 4.2V before the actual balancing can be performed (this is commonly called as top-balancing).

Googling for an active balancer solution lead me to expensive proprietary solutions that I cannot afford at the moment.

Hence, I decided to give it a go by designing it myself using low cost parts.

This is what I come out with:

The schematic.

The principle is quite straight forward really. All I did was converting the synchronous buck converter module (MP2307) to become a voltage follower (look at R4 and R5). In this case, the voltage that it is following is the half of the battery voltage.

In theory, I can make the power converter module to follow any voltage that I want (within limits of course). This is a very simple way of modifying the voltage regulator to instead of having the voltage to be set by an internal voltage reference (0.925V in this case) and a resistor-network, this technique forces the chip to follow an external voltage reference instead.

In other words, the circuit forces the voltage regulation to be governed by an external voltage reference, instead of the internal one.

The circuit constructed on a perfboard.

I originally intent to manufacture and sell on it Tindie, but, before I do that, I thought it would be nice to have a review by a friend of mine in the community (Vanarian from Endless-Sphere). 

Unfortunately, the board that I posted to him in France was lost during shipment and the board does not reach to him.

I lost my motivation and decided to open source this instead with the hope that the community can benefit from it and made some enhancement to it.

I believe this is the first of its kind: simple and low-cost. Maybe someone can prove me otherwise? ;)

The design files can be found in the links below.

Github: https://github.com/AfdhalAtiffTan/LowCostActiveBatteryBalancer
Endless-Sphere: https://endless-sphere.com/forums/viewtopic.php?f=14&t=87372
Hackaday: https://hackaday.io/project/20825-low-cost-non-dissipative-active-battery-balancer
Banggood: https://www.banggood.com/5Pcs-Mini-DC-Adjustable-Power-Supply-Buck-Module-Step-Down-Module-p-952402.html?p=3117141642416201505D
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