Ah OR Wh

[awol]

My 36v 10aH battery is well used and nearly expired.
Today I stuck my WATT meter on my battery for the first time in quite a while today. It told me my regular journey used 3.37aH or 117wH. (most of the journey was at about 32v too)
In theory, if I were to replace the battery with one which is only just big enough for my journey, (ignoring max C ratings for max amps) would I need a 36v 3.37aH OR any volt's being 36,44,48 which works out to 117wH ?
I am not going to do this, I'm just interested in whether it's the aH or wH that matter.

 

[danielrlee]

As long as you are travelling at the same speed, over the same journey, with the same amount of headwinds, 117Wh will get you there. The trouble is that by raising the voltage, your top speed increases and unless you are disciplined enough to stick to your old top speed, a new higher top speed will use more energy.

 

[Deleted member 4366]

You need one that does 20 amps and 117 Wh. Any volts would do, but it's easier to use more power with higher voltage.

 

[Danidl]

You need one that does 20 amps and 117 Wh. Any volts would do, but it's easier to use more power with higher voltage.

I hate to disagree, but voltage matters. If the motor were purely electromechanical then the extra voltage would either cause it to just speedup (DC motor with brushes) and the extra voltage would become a source of heat. If the voltage is excessive it will cause stress on the insulation on the windings and premature aging. I f the motor is of the brush less type, then it is being powered by a DC to AC 3 phase electronic conversion circuit and the problems can be more severe. There will be additional heating and voltage stress on the power components with significant ly reduced life.
The fact that in a individual case one might get away with it for a time, does not negate the fact that it would be bad practice. All engineering design involves compromises and the economic manufacture of consumer products means that components are selected which will be stressed a little, however increasing the stress decreases lifetimes.

 

[Deleted member 4366]

I hate to disagree, but voltage matters. If the motor were purely electromechanical then the extra voltage would either cause it to just speedup (DC motor with brushes) and the extra voltage would become a source of heat. If the voltage is excessive it will cause stress on the insulation on the windings and premature aging. I f the motor is of the brush less type, then it is being powered by a DC to AC 3 phase electronic conversion circuit and the problems can be more severe. There will be additional heating and voltage stress on the power components with significant ly reduced life.
The fact that in a individual case one might get away with it for a time, does not negate the fact that it would be bad practice. All engineering design involves compromises and the economic manufacture of consumer products means that components are selected which will be stressed a little, however increasing the stress decreases lifetimes.

None of that applies to low voltage ebike batteries. You might just as well say that his bike will catch fire if Birmingham gets hit by a 1 km asteroid.

 

[Danidl]

None of that applies to low voltage ebike batteries. You might just as well say that his bike will catch fire if Birmingham gets hit by a 1 km asteroid.

We must agree to differ.

 

[D8ve]

We must agree to differ.

A 1km asteroid will obliterate everything, setting the bike on fire is relatively minor effect for that one.
As is using different voltages. The speed is electronically limited as is the power draw on most Ebikes. The motor is capable of dealing with a reasonable range of voltage and current combos. So I'm with D8veh on it's not too significant. If you are pushing the envelope at all then it is important, 100 volt batteries and 63 volt capacitors is not good, and efficiency will drop but on our gear few people will notice the difference.

 

[awol]

Based on similar calculations a year or so ago I had purchased 6s+4s worth of 6aH rc-Lipo batteries, presuming this should have equalled to 42v x 6a to give me 252watts of total power.
My journey used aprox 120w each way 240w total, so this battery should be ok, right?
Nooo....
Today I tried my 6a Lipo battery for the 1st time and I got there and not even half way back before I was warned to stop.( I forgot to plug on my watt meter too)
To save me wasting the 10s Lipo-cheapest option, can I add another 2s 6ah to give a total of 302w @ 12s 50v (still 6aH though) to get me there and back.
Based on the help above I'm pretty sure it will work but before buying just want to check.

 

[D8ve]

Ah you did the calculation on full charge batteries. 42 volts.
It starts at 42 then falls to 32 at dead flat, so using 32 *6 ah it's a 190 watt hour battery!
Given the need for some leeway use low voltage for the calculation and your 12S is a 38.4 volt 6 amp battery. It is cutting it fine, no leeway for headwinds etc.

( I didid the math in head so errors are expected)

 

[anotherkiwi]

Based on similar calculations a year or so ago I had purchased 6s+4s worth of 6aH rc-Lipo batteries, presuming this should have equalled to 42v x 6a to give me 252watts of total power.
My journey used aprox 120w each way 240w total, so this battery should be ok, right?
Nooo....
Today I tried my 6a Lipo battery for the 1st time and I got there and not even half way back before I was warned to stop.( I forgot to plug on my watt meter too)
To save me wasting the 10s Lipo-cheapest option, can I add another 2s 6ah to give a total of 302w @ 12s 50v (still 6aH though) to get me there and back.
Based on the help above I'm pretty sure it will work but before buying just want to check.

D8ve is correct. Are these Multistars or cheaper? You have a 6 Ah x 80% actual available = 4.8 Ah x 37 V battery which gives you 177.6 Wh. Graphenes are about 90% of nominative Ah which is pretty good. I have had about 85% from my Multistars but the alarms were ringing and I was down to 3.5 V per cell.

Best bet is another couple of batteries 2 x 4S + 2 x 6S in parallel and then those groups in series for 12 Ah nominative = 9.6 Ah or 355.2 Wh. Then you will have a comfort zone.

 

[awol]

Thanks for the replies, so it appears I got my calc's wrong, based them on the wrong voltage and did not know about the 80% discharge idea. I will plug the watt meter in over the next day or so for some readings.
The blocks are turnigy nano-tech and were expensive which is why I wanted to salvage them with maybe a matching 2s. I got them a couple years or so ago just before the multistars were available.

 

[anotherkiwi]

You were thinking peak voltage and not the average voltage you get from the packs if you watch your voltmeter you will see it drop very quickly from 41.5 (which you should charge to instead of 42) V down to 38 V then spend a very long time at 37 V before dropping to 36.5 where your alarm goes off.

nano-tech is also (like graphene) advertised as giving about 90% so about 199 Wh. If you want to double up the pack today's price is 137€.

Your 44.4 V pack idea 2S + 4S + 6S should give you about 239 Wh so you would just about get home. Can your controller handle 49.8 V peak? If so I would say go for that and pedal a little bit harder on the way home