running 10Ah system with 9Ah battery

[yyayin]

what happens if i use 9Ah battery, when the controller is set to 10Ah (or more)?
and are there any controllers, that can work with both 24V and 36V batteries, without opening it etc?
actually what i want is to have the option, to (sometimes) take a very light second battery in my bag.
so i thought about 24V; 9~8Ah, when my system is set to 36V and 10Ah.
(sorry if any English mistakes, i'm not native speaker)

 

[Ypedal]

"Ah"AmpHour, is a capacity measure, your controller uses " A " amps.

So you have a 9ah battery with a 10 amp controller.. this is ok.

example, if you use a 10ah battery with a 20 amp controller, the controller could draw 20 amps for 30 minutes. ( not realistic.. but for an example ) . ...

or, with the above example, if you are light on the throttle, you could 10 amps from a 10ah battery, for 1 hr.

 

[yyayin]

thanks Ypedal, that's encouraging
so if i got you right, i will manage, but pls see the whole thing:
i currently ride a system claimed to be 250W.
the battery is 36V so i guess the controller draws about 7Ah.
the plan is to buy a new controller, that will be set to supply at least 10Ah to the motor, in order to increase the torque.
sometimes i want to take an extra battery in my bag, to extend the range a bit.
i hope that battery can be 24V, and, let's say - 9 or 8Ah, and still run the bike with 10Ah controller.
and about drawing 20Amps for half an hour (out of a 10Ah battery) so why you say it's not realistic?
do you mean that if the difference is small so it's more likely to be possible?

 

[Ypedal]

I recommend reading :
http://endless-sphere.com/forums/viewtopic.php?f=14&t=26621

see " Peukert Effect "

 

[dnmun]

you need to measure first before buying new stuff. if you don't have a voltmeter, get one first. if you don't have a cycle analyst or wattmeter get one to use. there are cheap imitations of displays on ebay mentioned here but hard to read in daylight.

the motor will take as much power as you give it. the controllers have a current limit rating stamped on the cover. use that. get the wattmeter or cycleanalyst, it will make life easier, save you money, and keep you from being stranded a long way from a charging location. if you don't know cycle analyst, go to ebikes.ca and read there.

 

[yyayin]

ok thanks (to both of you).
i think now i got the picture:
24V, 8Ah - means at 24V the battery can supply 8 Amper for one hour.
and there is a range of "flexibility" (C-rate), so that battery can also increase the amps and shorten the working time accordingly.
am i right ???

 

[dnmun]

actually capacity is measured using a .1C discharge rate. if you discharge at a 1-2C rate then the capacity is significantly impacted by the peukert effect. that's why ypedal said to read about peukert effect.

it is worst for the SLA packs, but does have significant impact on lifepo4 too which you are using if i understand you.

 

[yyayin]

yes, i have read it but now with your explanation i understand better.
and yes, i use lifepo4 battery (if that means lithium) and not SLA packs.
so the capacity will be impacted, but it will however somewhat work, (nothing will be damaged),
that's ok.. as long as will extend the range a bit i can give it a try.
because 36V 10Ah batteries are much heavier, and i only need it for the last 3-4 km of the ride.

 

[dnmun]

if you have a ping signalab i can assure you that the BMS will prevent you from over discharging the cells. on the Vpower BMS and the green one from BMS battery, i cannot offer the same reassurance. the headway BMS will also protect your battery from over discharge.

you will know it is low long before that, but you should use a cycle analyst or wattmeter to keep track. but you have to keep the pack balanced in order to get the full capacity that you need to be able to depend on the watt meters. they count coulombs, not measure voltage.

 

[d8veh]

Why do you want to use an additional 24v battery? Is it because you already have one?

Most 24v controllers can work with both 24v and 36v batteries, but a 36v controller can't work with a 24v battery because it thinks it's empty and switches off.

An additional battery will help you go further, but not faster.

The average 250w controller is set to 14 or 15 amps. 250w means that it can give 7amps all day without getting warm. Most 250w controllers will draw 600w from the battery.

Generally you need a battery aH rating equivalent to your controller maximum amps x 0.66 as a minimum unless you have a special high discharge battery. This means a 10aH battery for a 15 amp controller is the minimum, but still you'll get voltage sag up hills, better would be a 15 aH battery. The further you go below the minimum, the greater the voltage sag and the more likely you'll get an early cut-off (when your battery thinks it's empty)

You can get a bit more climbing power by soldering the shunt in the controller, which increases the current limit, but the battery needs to be able to give it, so not really advisable if you have an old 9aH battery and 15a controller, but I've had good results as long as the battery performs like when it was new.

 

[dogman dan]

If I understand it right, you just want more range primarily.

You can paralell connect two batteries of the same voltage, so to extend range of your 36v battery, you need another 36v battery of the exact same type. It can be possible to mix types of the same voltage, but it gets tricky since one 36v may be actually 42v, while another 36v may be actually 46v.

Adding more ah to your battery capacity will not harm the controller. That's what the controller does for you, allows the set amps to flow to the motor, not too much, not too little.

Don't change the voltage by series connecting another battery, that controller wants "36v". Don't paralell connect a 24v with a 36v either.

Get a cheap digital voltmeter somplace, Harbor Freight has them cheapest in the USA. You need it to know what is going on with your battery at times.

 

[yyayin]

it's just for additional few km of range,
i want to take it with me in the bag and replace when the main one is empty.
(normally i don't need it, but from time to time can be useful).
i just wanted to know if it will work, and not damage anything...
so actually now i got the answer, it should be ok.
so thanks everyone!