Random Projects

I recently bought a toy car for my Son which has a 2S lithium battery and comes with a USB charger FTX Tracer Truggy. I couldn’t get the toy off him, so I bought my own monster truck version – but my charger had a flashing red light whereas his has green when charged with red when charging. So I thought mine was broken. (it wasn’t it was just a slight variation)

When I removed the case I was puzzled for a while how this works. The battery is a 2s battery which is nominally 7.4V. Normally if you want more than 5V you’re going to need an inductor or switched capacitor boost circuit. But there are none on here. Only a couple of tiny supply decoupling capacitors; so how does it work?

Why is it interesting?

Above is what happened in my living room a few years ago when charging a toy helicopter. This was a 7.4v/2s battery of 800mAh capacity. Even for such a small battery the fireball was huge and very hot. This is why I never leave them unattended. You can see that the battery charger has only 2 wires and so there cannot be any balancing. I now charge lithium polymer batteries in the kitchen – on the stainless steel stove top. That way, fire cannot spread, there’s a cooker hood to extract any fumes and in the worst case, the stove can be easily replaced.

Unlike older rechargeable batteries, lithium cells do not tolerate over-charging with the risk of spectacular explosions. So battery packs with a series (2s, 3s….) of cells can be charged in series but if one cell has a lower voltage than the others then serious problems result. Its not as easy as limiting the charge current with a resister like in the good old days of NiCads.

You need to switch from constant current to constant voltage at around 60% state of charge and each cell may have a different state of charge. Cheap chargers monitor each cell and terminate charging when any cell reaches 4.2V, so a cheap drill battery may seem to get less and less capacity until its useless, all you actually need to do is charge them all independently to restore its capacity.

A proper balance charger monitors each cell independently and prevents that an individual cell charging whilst continuing to charge the rest. This is not trivial because they are all still electrically connected together.

A proper balancer will provide a voltage that is at least 4.2V x <cell count> and whist charging will shunt each cell as it becomes full. This means that current bypasses the cell through the transistor Q1 – Q3 and continues to charge the remainers.

Shown above is a discrete (made from bits not a chip) self-balancing system. It uses a simple zener and transistor that starts to turn on as the cell approaches 4.2V, its adjustable per cell because components are never exact. Its great because it can’t really go wrong.

No Shunt, No voltage boosting

Taking a closer look at the charger, we see USB plug on the right with 5V available. On the left we see the 3 wires going to the cells, the centre wire being a tap between the cells. To the right of those wires we see 5 transistors. And the whole thing is controlled using an anonymous (no label) micro-controller. This left me puzzled for a bit but an hour later I worked out what it must be doing. Its charging one cell, then the other and the transistors are used to connect each cell to the charging “bus”.

You could do this one cell until full then the other – but if you were to then interrupt the charge, say in a RC car, then the flat cell would end up reverse charging and that would ruin it. A quick connection to a multimeter shows that the charger charges each cell for 1 second then switches to the other and back again continually until charged. This swapping batteries coincides with the flashing charge light. I think the charge current is limited by the large 100 Ohm resistor, and the micro controller will also be able to measure that. Since USB is 5V and a fully charged battery is 4.2V that leaves 0.8V to be dropped by the transistors and the current limiting resistor, which you can just about do with normal transistors which have a voltage drop Vce of about 0.3V. So that would be 0.3+0.3 leaving 0.2V drop on the resistor.

Overall, its a clever design and may have been obvious, might even be common with cheap toys that require more than a single cell – but I hadn’t seen it before.

My concern is referring to the charge vs. capacity graph – when charging and relaxing as opposed to continual charging, I think its much more difficult to detect when the battery is at 100%. So it probably doesn’t optimally get the last 10%. Having said that, pulsed charging and charging to 85% is usually good for the battery. In an EV or Hybrid Vehicle for example, the battery takes charge and discharge intermittently at any state of charge.

Another concern is that the micro-controller runs a software which can easily glitch or get stuck. You’d hope the software had a watchdog reset that resets it if it does but if it doesn’t then you could find that the circuit dumps 1.2A into one cell until it explodes. That cannot happen with the discrete design, for safety you probably want to see that the cell voltage can never exceed 4.2V but doesn’t look like here is any stuck protection, unless those big blobs that look like diodes on the output are 4.2V Zener diodes, such a thing does exist – but they appear to be marked ss34 which is basically a 3A shottkey diode to stop the charger being `driven` from the batteries.

Finally, if a lithium cell is over-discharged (below 2.5V per cell) you cannot whack a full charge current on to it to start charging, you need to charge at a low rate until the voltage recovers, then resume charging. Similarly, you must also take care charging hot batteries, its better to reduce the current or wait for it to cool before charging. A proper BMS (battery management system) will typically have temperature monitoring for that reason.

For all its cleverness – probably best to never leave it unattended.