Battery Fuel gauge for use with swapable batteries

I’m looking for an battery fuel gauge IC for removable batteries (think Dewalt tool battery). The battery pack is 20V (5s). I would like something more accurate than voltage monitoring. As a starting place I’ve selected Ti’s BQ34Z100-R2. It appears that these fuel gauges are expected to be installed inside the battery pack. I need a fuel gauge that is external to the battery and capable of reading the battery capacity without knowing the C rating, or battery capacity (user may choose a smaller or larger battery on the fly). I think the BZQ34Z100-R2 could still work, but if someone could confirm it;s function for my use case or suggest another IC that would be appreciated.

Hi @jimmy1 sadly, that’s not how these types of devices operate. While the Ti’s BQ34Z100-R2 is quite complex, thus it’s not fair to simply state:

These Fuel Gauge IC’s need to know how much “C” has entered the battery or left the battery over time, and know the type and composition of the battery…


If these devices, don’t work the way I need, can you suggest another. Or in your opinion am I stuck guessing battery charge state based on voltage?

Correct, without a coulomb counter, it’s a pure guess. While you can use the voltage to know when the battery is close to empty.

As Li-ion stay pretty flat, till they drop off a cliff:


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Thanks, I’m aware of the discharge curve of the li-ion battery and complications of use only voltage to guess the battery capacity. I was hoping someone smarter than me actually had a solution for reading the remaining charge of an semi-unknown battery.

This isn’t possible in any meaningful sense.

Voltage measurements can give reasonable estimates of proportional state of charge (e.g. “it’s half full”) but translating that into an absolute quantity requires knowing the container’s capacity; half of a 1 gallon can is different than half of 5 gallon can.

Moreover, usable battery capacity can change depending on things like temperature, rate of discharge, cell health, the chosen end-of-discharge criteria, etc. Even in a best-case scenario with built-in monitoring it can be a moving target.

I’m not worried about total amount of charge left. All I’m looking for is an indicator for the user where the battery level is. If I could get a battery percentage with +/-5% error that would be fine.

@jimmy1 are we looking at ‘raw’ cells or a module like a drill battery pack?

If it’s a drill battery pack, there is a pcb/board inside that pack that isolates the raw cells from the world. Thus you are only going to get the “20V” value or it’s rated voltage till it dies from the drill connector leads…

@RobertCNelson Its an isolated battery pack (like a Dewalt tool battery). I’m not sure why you think the battery pack will deliver its rated voltage for the entire duration of the battery life, this is simply not the case. The only packs that I’m aware of that work like that are the power banks which have a regulated 5V output and those are not technically a battery pack, but do have one inside of them.

@RobertCNelson The PCB inside battery pack are typically battery protection (and sometimes LED charge indicators). Typically they function to disconnect the battery from the outside world under certain fault conditions (overcharge, over discharge, over current, over heat, etc).

Yeah, good point, it really depends on the manufacture of battery pack and what ever extra features they add to their design… From usb charging ports, to led indicators.

Short answer is you’d have to factor the voltage of the battery to the capacity & chemistry of the battery. When you swap batteries, are they the same voltage & chemistry, just different capacities and C (discharge rate)?

A lot of the lithium batteries on the market today for cordless tools use a multi-prong electrical connection on the battery, and the charger is responsible for balancing each of the cells up to a certain voltage depending on the rated voltage of the battery. A lot of these chargers use constant current charging for the initial stage, and then roll over to a “voltage balancing” or “topping” stage for the remainder of the charge. Most finished or store bought chargers will detect cell voltage before charging begins to verify the cells are safe for charging by doing a simple voltage check of each cell to make sure the cells are not under or over volted based on the cell type, along with comparing the cells making sure the cells are within a few milli-volts of each other, otherwise the charger won’t charge as a safety measure.

Some chargers are able to charge various battery capacities within the same voltage rating, such as a 20v 2500mA and a 20v 5000mA. By doing a simple voltage reading of the cells (or entire battery) will give a basic approximation on how much battery charge is left in the battery in relevance to the specific battery capacity, and factoring that in with the exponential battery voltage discharge curve for the specific chemistry of battery. There are chargers that are able to charge a set amount of capacity, but most likely are unable to determine the capacity of which the battery already holds or is capable of holding unless some preset values are loaded or selected externally through a selector switch letting the gauge know it is a different battery. One could possibly formulate a circuit that would give an overall charge representing the capacity and voltage as some kind of sum, but this would require some hefty engineering and would only be an approximate value.

An easier solution 1597-1237-ND would be an example volt-meter one could mount to the device to read voltage across the battery (all cells), once you find out what voltage each of your battery packs fizzle out at, you could make a chart, or put a label on each of the battery packs indicating when the battery is low, or will shut down. It may not be ideal but is an alternative option. Just make sure this is not connected to the battery or charger when charging.

@Ryan_2724 Thanks for the suggestion. All the batteries will be the same voltage (20V), but possibly varying capacity (3Ah, 5Ah, 9Ah, for example). One of my co-workers recently pointed out MAX17263 which looks promising.

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