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PreacherG 11-09-19 05:48 PM

Batteries
 
I understand different voltage and how multiple cells produce the desired voltage. My question is on battery capacity. Does a 10 Ah battery deliver 10 amps at the specified voltage, i.e. 48 V, 10 Ah will deliver 480 watts, then is dead? V x A = W
Or does the mean it can deliver power at a RATE of 10Ah, or 166 ma / min? For how long?

In other words, will a 48V 10Ah battery power a 250W motor running battery only full power for about 2 hours or a 500W motor running full power for about 1 hour? Clearly, the 500W would be much faster.? A 36V 10Ah battery would power the 250W motor for 1 hours?

Can I determine the amount of power stored in a battery by multiplying V x Ah = Watts? Is this not a better determination of range, because the bike will draw the same wattage for the same riding conditions, up to the max motor rating?

I know lead acid batteries only allow about half the available power to be used before the battery begins suffering permanent damage, so a 480W battery really only has about 240W available to use. How far down can a Li battery be drawn down without damaging the battery?

If I get a bike and I am not happy with the range, is upgrading the battery size a simple process? Do electrical connections have a standard? How about mounting?

As always, Thank You. Your wisdom is appreciated.

Doc_Wui 11-09-19 10:17 PM

Di you understand that batteries are made up of groups in series? Like 48V is 13 groups, and 36V is 10 groups? And you know that a group has at minimum one cell, but usually 3 to 5 cells.

The AH rating of an ebike battery is simply the Ah rating of each cell x the number of cells in the group. Take a 40 cell 36V10AH pack. We know it's 10 groups, so 4 cells per group. We can divide 10AH by 4 to learn it uses middling quality 2.5AH cells. Similarly a 52 cell 48V10AH pack could also use the same cells.

Now that you have digested the above, the WH rating is the batteries AH rating x nominal voltage. A 36V10AH pack is 360WH. A 48V10AH pack is 480 WH. If the first pack had 40 cells, the second one will have 52 cells. Those extra 12 cells give you another 120 WH.

Yes, if (and only if you maintain the same speed), a 36V bike and a 48V bike use the same WH per mile. So a 48V10AH pack does give more range than a 36V10AH pack. More speed too.

In general, you can't get the full WH out of a pack unless you're willing to kill it. The AH rating on a bare cells is determined by testing it from full charge (4.2V) down to its safest low charge (2.5 to 2.7 volts). For reliability, a good ebike controller shuts off a battery when the average cell voltage is 3.0-3.2 volts. So you might leave 10-20% of the available WH still in the cells. If you put a dumb light bulb on your ebike battery, you can run it down til it's flat and it might not be able to recharge.

If not happy with the range, yes, buy a second battery with more WH that fits, or buy a spare battery and carry it. Or ride slower.

PreacherG 11-10-19 05:05 PM

Thanks Doc. I did know about how batteries were built. You have confirmed my idea of battery capacity (V X Ah = watts) and thanks for the info about 80% discharge is as far as they will go without damage.

I assume most controllers have circuitry to protect the battery from excessive discharge, but it is wise to not trust them too far.

fly135 11-10-19 07:39 PM


Originally Posted by PreacherG (Post 21201994)
I understand different voltage and how multiple cells produce the desired voltage. My question is on battery capacity. Does a 10 Ah battery deliver 10 amps at the specified voltage, i.e. 48 V, 10 Ah will deliver 480 watts, then is dead? V x A = W
Or does the mean it can deliver power at a RATE of 10Ah, or 166 ma / min? For how long?

In other words, will a 48V 10Ah battery power a 250W motor running battery only full power for about 2 hours or a 500W motor running full power for about 1 hour? Clearly, the 500W would be much faster.? A 36V 10Ah battery would power the 250W motor for 1 hours?

Can I determine the amount of power stored in a battery by multiplying V x Ah = Watts? Is this not a better determination of range, because the bike will draw the same wattage for the same riding conditions, up to the max motor rating?

10Ah means it can deliver 10 amps for an hour or 600amps for a minute. However, it really won't deliver 600 amps for a minute, and it may not even deliver 10 amps for an hours. Amp-hours are normalized to an hour, but the actual capacity might only be realized at a lower current draw. For example a 10Ah battery might deliver 5amps continuous for 2 hours, which still equals 10Ah. So the battery capacity might be realized running the 250w full bore, but maybe not the 500watt. The answer would be to look and see at what continuous load a battery's capacity is measured. They might actually deliver 10 amps for an hour.

V x Ah = watt-hours, and yes that's the actual energy capacity vs. Ah which doesn't give you the energy without the voltage specified. Not sure how bad it is to run them empty. I believe the controller will shut down and protect the battery on low voltage.

TonyVov 11-10-19 08:24 PM

PreacherG

Battery energy capacity which is the aH rating x Voltage is not referencing the power but the energy.
The 18650 cell format size comes in many different internal design variations that can give different performance for both the maximum power (the maximum discharge current). The energy is expressed in aH. The power in amps or C

One 18650 cell typically has a chemistry delivering it's aH at 'nominal' 3.75V going from 4.2V at full charge to 2.75V at full discharge or empty. This is the energy capacity. It is generally listed on the spec in maH Milliamphour for example 2600mah. If you put 4 of these in a row you get 10.4aH, many eBikes use this. If you put 5 of the same cell ina row you get 13aH and 25% more range.

If you take the row of 4 batteries and stick 13 of them end to end you get the 48V nominal battery you often see. Different chemistry will deliver different nominal Voltage. This type of pack would be 54V full charge and around 35V when discharged.
Putting 14 in a row gives 53V nominal.
The real energy in wH is the aH x the voltage but as the voltage varies a lot as a function of charge level and also discharge power the ah is what manufacturers count and quote.

So for the same Voltage you can get more energy by buying a battery with more aH.
You can get more aH by buying more batteries in a row example buying 13sx4p going to 13sx5p with same chemistry gives 25% more energy or by buying better chemistry 3000mah cell vs 2600mah.

For an eBike get the range from the aH but not the power
The power is measured by a different cell specification. The discharge rate often quoted in amps or in 'C' which is multiples of the nominal ah rating of the cell.
If you drain a battery at less than 1C 2.6amp for 2600mah cell it will last many cycles. If you drain it at 4C, 10amp it's life will be shorter. Each cell design had it's maximum recommended discharge rate.

The designer selects the cell to give enough amps and power for the application. For a high powered mountain climbing beast you want lots of amps and have a motor or controller to suit. For this you need a cell with very high discharge current capability which often don't last as long. Radio control devices also like lots of current For a long range commuting eBike range is more critical and designer will focus on ah not discharge rate to give you best range You need enough amp to meet the thirst of your motor and controller. There are literally thousands of different varieties of ah and discharge rate capability out there and the battery used in your device is selected for the application needs.

So to review a battery pack, learn it's construction (# in series x # in parallel) eg 13sx4p
Compare to the quoted voltage and aH or wH rating and you can guess at the cell spec.
Check the maximum discharge rate.

If possible get the cell specific info
Learn its cell and chemistry if you can (example Samsung 18650 2600mah)

The are many online independent test reports for individual cells where you can then check your devices characteristics during charge discharge and life time cycles.
You can also estimate the amount of sag or voltage drop you would expect under load.
It is well worth doing your research as there are many marketing ploys out there to trip up the uninformed buyer.

If you change designers selection be ready for surprises. Your controller and motor may not be capable to eat what you throw at it. For example adding extra aH at same voltage generally will also increase Max amps.
Adding extra voltage (series) may give you more power and range but your controller may drain it too far reducing life and increasing chance of destroying your pack.


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