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Battery Monitor / What am I looking for (does this even work?)

Old 06-22-2010, 12:42 AM
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mclarkson
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Default Battery Monitor / What am I looking for (does this even work?)

I recently received this inexpensive battery monitor from HobbyKing. I understand that it is not terribly accurate - reviewers all say to ignore the last digit (100ths/volts) - but I just wanted something small to do a quick check in the field and make sure I'm good for another flight (or not.)

I have a few problems, though.

A) I'm not sure the unit I got is working properly, but I am very ignorant. When I check my Radian's stock 3-cell battery, charged up 3 days ago and sitting since then, I get 4.17 on each cell for a total of 12.5. The specs on the battery are:
11.1V, 1300mAh (14.43Wh).

Likewise, the stock 2-cell batteries on my Super Mini Cub - 7.4V, 300mAh (2.2Wh) - read between 4.17 and 4.2 per cell, for a total of around 8.4.

Shouldn't the batteries have 11.1 and 7.4, respectively? Or is it my ignorance showing?

B) Whether this unit works or not, I'm not really sure what I'm looking for. How low is too low? I've read lots of dire warnings about letting the voltage drop too far. I think the Radian's ESC will save the battery by killing the motor, but I don't know where/when that will happen. Where should I stop flying it? 7V? 5V? 4V?

Thanks for any info.

Last edited by mclarkson; 06-22-2010 at 01:19 AM. Reason: spelling
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Old 06-22-2010, 12:49 AM
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crxmanpat
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A) A fully charged lipo cell will read 4.2 volts. So yes, your monitor is working correctly. So a fully charged 3-cell lipo will read 12.6 volts total.

B) On a 3-cell lipo, 11.1v is fully discharged. And going below 3 volts per cell is definitely a no-no.

With a plane such as the Radian (powered glider), flight duration will largely depend on how much throttle use there is. I'm sure you could do a dozen short zoom climbs to altitude, then glide around until you need to zoom back up again.

I would say anything 3.8 volts and below you need to recharge.
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Old 06-22-2010, 12:58 AM
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flydiver
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Yep, your ignorance is showing.

7.4 and 11.1 are known as [nominal] voltages.
Fulling charged lipos will be 4.2v/cell. Over that is bad and rapidly gets dangerous. A bit under is fine.

For resting voltage try not to go below 3.5v-3.7/cell resting. If you go below 3.0v/cell resting your Radian charger won't even accept the battery and you have done some damage.

The Radian ESC has an LVC of 9.2v WORKING (under load). If it hits LVC and you check it RESTING it should be more like 10.5v. Preferably for lipo longevity do not fly to LVC. It is not terrible on the Radian ESC but for some brands (GWS, Electrifly) it's awful and battery destructive.

Lipos do not have a linear discharge curve-it's like this
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Old 06-22-2010, 01:16 AM
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mclarkson
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Alright, thanks guys. I appreciate the info. I'm used to 'normal' batteries where a 1.5V cell shows 1.5V when full, not when empty.

I'm pretty sure I'm not using anything like all of that Radian's battery, but I'll be interested to see what's what.

Danke
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Old 06-22-2010, 01:52 AM
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flydiver
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Originally Posted by mclarkson View Post
I'm used to 'normal' batteries where a 1.5V cell shows 1.5V when full, not when empty.

Danke
NiXX re-chargeable are so called 'nominal' 1.2v batteries, but when fully charged are more like 1.4+ and at 0.9 are damn near dead. They have a similar S-shaped discharge curve to lipo.
Alkalines have more of a linear discharge.

You have to learn a whole new set of rules for rechargeable batteries of about any sort.
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Old 06-22-2010, 02:02 AM
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mclarkson
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Okie dokie. I'm new to all this (first R/C flight about a month ago). I appreciate all the info.

I'm assuming that the 3.8V/cell rule would apply to any LiPo battery?
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Old 06-22-2010, 02:25 AM
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flydiver
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Yup. The higher the C-rating the better the battery holds power to the end > BUT, when it hit the end it's dead FAST!
Older, low C lipos were a little more forgiving. Many ESC still have an LVC stuck in old lipo technology.

You may enjoy some reading here.
Basics every battery user should know
Great site. Take it a little at a time.
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