**How many amps can a battery put out?**

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Birchy85
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**How many amps can a battery put out?**

Post by Birchy85 »

hi there,

if you have a 12v @ 10A lead acid battery, if you hooked an electric motor up to it, and put it full throttle with a full load, so the motor has to draw max current from the battery, would the battery end up putting out just 10A or would the peak be much higher?

white zombie (the electric car), as stated on the website said the motor at full throttle was drawing 2000amps from a battery pack which was rated at 960amps.

is this where the concept of 'cold cranking amps' comes from, etc etc? as in, does that have something to do with this?

I would really appreciate anyones feedback on this one.

Cheers! Image
antiscab
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**How many amps can a battery put out?**

Post by antiscab »

Hi Birchy,

Some batteries have a cold cranking amps rating, this is an indication of power.
This is the max number of amps the battery will give, when its at a temperature of 0c, before voltage drops below 7? V
At higher temps, the max amps is much much higher.

Most batteries are rated in AH. This is an indication of capacity.

Usually these batteries are cranking batteries, unless someone else has used them successfully in an EV before, dont use them.

The EV type batteries have a listed AH rating and C rating.
multiply the two together to get A.
Multiply by voltage to get power. (remember to account for voltage sag under load).

For instance, for my own pack:
288v, 90cells, 90AH, 5C max discarge, 0.2vpc drop per C load.
Max current is 450A.
Voltage at 450A is 2.2vpc.
Therefore: Power=450*2.2*90=89100w or 89kw. 89hp by the time you get it to the wheels.

Matt
Birchy85
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**How many amps can a battery put out?**

Post by Birchy85 »

thanks matt, your my savior today lol
what if you using lithium batteries, they wont have a CCA rating. how do you determine their max amp output?
say you have 20 12v @ 20A batteries for example, and they had no CCA rating...how would you work it out?

thanks again for all your help! Image
antiscab
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**How many amps can a battery put out?**

Post by antiscab »

lol no worries.
If you are working out how much power you can pull from a battery in an EV environment, then dont worry about CCA.

if you have 20 x 12v 20AH batteries, then it is limited by one of two things. either:
1) The manufacturer has a rated max current above which battery damage occurs

or

2) the current at which the voltage drops to half its resting voltage.

Its the lower of the two.

If the manufacturer doesnt rate either voltage drop, or a max current rating, and noone in the EV world has used the battery before...walkaway.

The other side of the battery equation is useable energy.
Again, ill use an example from my own situation.

On my bike, the average current draw is 50A.

When i was running 5 x12v 20AH lead batteries, i had a usable energy of 60v x 12AH=720 Wh from a battery weight of 35kg. this gave me about 10-15km range.

i no run 19x 3.2v 40AH lithium cells. i now have a usable energy of 57v x 28.8AH = 1641wh in 30kg. this gives me a range of 40-50km.

To work out energy capacity of a battery you need to work out what your continuous average current draw will be, work out what voltage each battery will deliver this at, and what AH capacity it will have at this discharge rate.

again, another example.
The greensaver 12v 20AH 7kg lead acid battery at 50A has a voltage of 12v (a bit higher i think, its been a while since ive used these, Mal might have an idea) and a usable capacity of 12AH (15AH at the 50A rate, but you can only use 80% of this for decent service life). so 144wh per battery.

increase the average discharge rate to 100A, and the voltage falls to 11.5v, and capacity to 9 AH. 103.5wh per battery.

now, 20 x 12v 20AH batteries of the lead variety will weigh around 140kg, which on a bike is huge. If you need this much energy, i highly suggest lithium.

Matt
Last edited by antiscab on Thu, 31 Jul 2008, 19:37, edited 1 time in total.
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commanda
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**How many amps can a battery put out?**

Post by commanda »

antiscab wrote: i now run 19 x 3.2v 40AH lithium cells. i now have a usable energy of 57v x 28.8AH = 1641wh in 30kg. this gives me a range of 40-50km.

Matt


Matt,

Can you explain how the 40AH lithium cells become 28.8AH usable energy?

Amanda
Evt-4000 (Big Lithium grin)
Chopper pushbike 180 watt Cyclone
antiscab
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**How many amps can a battery put out?**

Post by antiscab »

Hi Amanda,

The capacity at the 1C rate on the lfp40aha units is 36AH.
for best cycle life you should'nt discharge further than 80% of this.
As part of my testing i was on occasion discharging further than this.
This resulted in me killing a cell by over discharge, by pulling all 36AH out. (so technically im now running 18cells:p and have a lower top speed and shorter range.)

This one cell as probably at a slightly lower SOC to the rest of the cells, or had a slightly lower capacity.
with the 40AH cells and rod dilkes BMS at www.evpower.com.au all the cells are within 1-2AH of each other.
1-2AH at near 100%dod can mean the difference between a healthy battery and a door stop.

A few notes with determining usable energy (and range):
the average voltage of a pack decreases with a higher average C-rate.
so if you were to double the size of a pack, you will get more than double the usable capacity.

In colder weather the voltage drop is higher (as much as a volt between 19cells).
colder air has a greater density and requires more power to push through (since theres more of it) so your winter range will be less than your sumer range.
How much of a range difference you will experience is vehicle specific, however ive found that on a moped this is very significant at 60kmh (the speed my moped speeds most of its time doing).
My top speed in winter is lower, and i need more A to maintain it.
My going to work Ah usage is around 8-9AH (57v) during summer and 11-12AH (56.5v) during winter.

Matt
Last edited by antiscab on Sun, 03 Aug 2008, 05:16, edited 1 time in total.
Birchy85
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**How many amps can a battery put out?**

Post by Birchy85 »

as batteries are rated AH around the time the resting voltage drops, when the amp rating is close to being the lowest...are electric motors rated in a similar fashion? or are they rated at their maximum amp handling capacity?

Once again, thanks for all your advice!
antiscab
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**How many amps can a battery put out?**

Post by antiscab »

almost,

batteries are rated in AH at 100% depth of discharge when the amount of active material on the plates has been consumed.
It just so happens that on lead acid batteries (aka our starter batteries in our cars) the terminal voltage is much lower than when fully charged.

This is true for most chemistries, the difference is by how much.

Electric motors are rated at their max continuous current handling capacity.
some have 1hr ratings and 1 minute ratings and so on.
in general, you can push a motor as hard as you like, until the motor temperature reaches a point where you have to cut back to its continuous rating before damage occurs.

The peak power on the series DC motors in most conversions is limited by what the commutator can with stand.

The peak power on AC setups (both PM and induction) is the current at which the motor goes into saturation, and the rpm at which the controller can no longer supply the amps.

Matt
Birchy85
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**How many amps can a battery put out?**

Post by Birchy85 »

what exactly do you mean by saturation? i'm not too farmiliar with this term.
antiscab
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**How many amps can a battery put out?**

Post by antiscab »

saturation is where the part of a magnetic field has reached an upper limit.
This is a limitation of all motors, particularly AC induction motors as these require the magnetic field to be a specific shape to work efficiently.
When the field starts to saturate, this field shape changes, and efficiency plummets.

Matt
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