Voltage reading question

MM1992 · Jun 30, 2017

I usually land within about 30 seconds of the voltage readout hitting 14.5V As expected, as soon as I'm on the ground and kill the motors, I'll watch the voltage rise back up to around 14.7 or 14.8 since theres much less current draw.

But the one thing I noticed recently, was this idle voltage reading is still lower than what I read if I check the battery with a multimeter. I was trying to run one down to 15.4V to store it. I put it in the H and turned it on, and the controller read 15.4V but I had tested it at 15.6V. This was with the motors off. I let it sit for a while until the controller read 15.2V and when I tested the battery, it was at 15.4V.

So is my multimeter wrong, or does the H read batteries low?

Tuna · Jun 30, 2017

When the H is on and transmitting telemetry back to the ST-16, it will be drawing power which will naturally drop the battery voltage a little bit. Your volt meter doesn't do that, so will read higher.

Rayray · Jun 30, 2017

MM1992 said:
So is my multimeter wrong, or does the H read batteries low?

After a period of time charging, flying, etc. I think the craft voltage reading on the ST16 probably is a little higher than actual.

My 2 chargers also read a bit higher than actual and my stock charger finishes at about 16.6 actual This may be by design. It has not affected my flying so I let it be. Charging no higher than 16.6 is a good thing in my opinion and I try to fly down to 14.5 or 14.6 dynamic, which gives me about 17 actual for storage.

That's a good post. Thanks.

MM1992 · Jun 30, 2017

Tuna said:
When the H is on and transmitting telemetry back to the ST-16, it will be drawing power which will naturally drop the battery voltage a little bit. Your volt meter doesn't do that, so will read higher.

Right, I would expect there to be a similar effect in voltage drop, but with the motors drawing so much more power than the other components I would expect it to be much more nominal, rather than nearly the same.

PatR · Jun 30, 2017

You will experience several voltage levels with a battery.

Highest will be just after it comes off the charging cycle. That's a somewhat false "hot charge".

Cold state full charge voltage when first inserted into the H and powered on. Of course that presumes you let a battery cool after charging before use.

Voltage under load/in flight. If you stop and hover for a moment to check voltage before landing you could start a low voltage tracking record.

Post landing voltage after motor cut off. Typically about 0.3v higher than under load voltage after landing. The motors at idle draw little current. Often the voltage at idle will be the same as immediately after motor shut down.

Post flight cold voltage after the battery has cooled after a flight. This voltage will be several 1/10s of a volt higher than the no load voltage.

There will most always be some difference between a volt meter and what is displayed in the ST-16. Unless both the volt meter and -16 are sent out for periodic calibration and certification they have room for small differences. A 1/10of a volt or so difference is not a big deal.

The two voltage levels you need be most aware of are those when a fresh battery is inserted in the H and the under load voltage. Should you land and elect to use a battery again before recharging the previous under load voltage is what should be used to establish battery state. Although the voltage bumped up a little after cooling it will very quickly fall off as it warms up under load. VERY quickly.

How an H lipo works is no different from any other lipo, but most likely haven't paid much attention to batteries before. If you want to have some fun, try checking the actual voltage of the 12v battery in your car sometime[emoji4]

kai_bcn · Jun 30, 2017

Hi all,
so is this a reliable way to check the batteries (until adapter gets here)?

PatR · Jun 30, 2017

Doing the math will of course provide individual cell levels which will help you understand how well the charger you are using balances the cells. Check them immediately after and about 1/2 hour after taking the battery off the charger. Looking good with what the pics are saying.

kai_bcn · Jun 30, 2017

Thanks PatR, so I could fly for 12min for example and check again to see what the individual cells are down to and hopefully they are equal then , too, correct?

PatR · Jun 30, 2017

You could, although a reading from a hot battery does not tell all you might want to know. Personally, I would not pull batteries mid flight to check individual cell status. Realistically, if you want to check individual cell levels just do it after a charge and before you use the battery. You can get much too spun up trying to check batteries at numerous points in their use cycle and end up flying little but driving yourself nuts keeping track of everything. Unless you are doing the same steps with 6 or 8 batteries of a known cell quality you won't learn all that much. One or three batteries of unknown cell quality is both too small a sample and leaves a considerable amount of base line info out of the process. What you want to know is the state of the batteries as they are to determine if all is good or if there might be a problem with a battery to avoid using it again.