Battery indicator goes down on a hill climb but then goes ba

[rmanalan]

(My post title got truncated... "Battery indicator goes down on hill climbs, but goes back up on descents or flat... Why?")

I'm trying to get an accurate gauge on how much battery charge I really have left -- it's hard to gauge when it goes up and down during a ride. After a 10m ride on hilly terrain (36v/10ah/bbs02b/500w), my c965 tells me that I have 4/5 bars left, but during a hill climb on my ride, the display showed that I had 2/5 bars left. Which one do I trust?

Does the battery indicator go strictly off of voltage?

 

[Drunkskunk]

Yes, the battery indicator goes off voltage, and your voltage drops because your amp draw from the motor increases. The harder your motor works, the more the voltage will drop while it's workinng hard.

There are 2 ways to explain this. One is with math explaining how ohms law relates to what is going on inside your motor and battery. But that's boring.
The other is with a simpler analogy. Voltage is the pressure pushing power out of the battery, and when your motor is using more power, you see a bit of a drop in pressure on the battery. It's the same idea as your shower going slack when your wife/girlfreind/roomate flushes the toilet.

 

[rmanalan]

Yes, the battery indicator goes off voltage, and your voltage drops because your amp draw from the motor increases. The harder your motor works, the more the voltage will drop while it's workinng hard.

There are 2 ways to explain this. One is with math explaining how ohms law relates to what is going on inside your motor and battery. But that's boring.
The other is with a simpler analogy. Voltage is the pressure pushing power out of the battery, and when your motor is using more power, you see a bit of a drop in pressure on the battery. It's the same idea as your shower going slack when your wife/girlfreind/roomate flushes the toilet.

Great analogy. Thanks. So, how should I determine percentage charge left on the battery? When the "pressure" stabilizes, I assume the voltage is going to be accurate for determining the charge left?

 

[brumbrum]

As long as you do not drop too much pressure that you hit the low voltage cutoff (lvc) of your battery that should be programmed into your controller or battery monitoring system, then when your battery is working on flat ground or after it has rested at a standstill will you see a true reading of your battery voltage.

Your battery will be designed to only be able to give out so many amps at one time so large voltage drop occurs when you are getting close to or hitting this.

 

[amberwolf]

So, how should I determine percentage charge left on the battery?

With a voltmeter (like what you have), accurate determination isn't really possible--you can make a good guess once you have enough experience with your system and various terrain.

But with a wattmeter, that counts watthours and amphours, then once you first test it to empty to verify it's actual capability, you can then use that as a pretty accurate guide (with most lithium types, at least), to how much capacity is used and how much left.

Cycle Analyst, Watts Up, Turnigy Watt Meter, and the zillion clones of the latter (itself a clone of the WU), can all do that. The CA would be the most accurate and adjustable / adaptable of them but it also costs quite a bit more.

 

[dogman dan]

Whether a crude bars indicator, or a voltmeter accurate to .001v, either way the accurate measure of voltage for guessing how much left is when you are stopped.

You can simply keep pedaling with the power off, don't have to actually stop. Just a 2 second throttle off may be enough, but your meter likely does some averaging so it may take longer with power off to show. So judging while you are coasting down a hill can be a good time to look.

Even with a very accurate voltmeter that reads instantly, it's still a guess. A watt meter is the way to really know. First you find out how big your tank is in real riding use, then you can judge where half of it is. A battery may be 10 ah, but actually have substantially less in real world use, how you ride it. But once you know your actual capacity is 8.5 ah, then you know you are half empty at 4.25 ah used.

 

[d8veh]

Your battery is probably not strong enough for your 500w motor. Unless you know different, it's probably only rated at 10A continuous.I was going to mention that after your post in the other thread. A 50 cell battery with branded cells would be better.