Lipo SLA boost?

[jimmyhackers]

name doesnt say it all.

bit of a weird one here.

running 48v 1000w brushless gearless motor on 4x 12v 20ah sla batteries. all is well.

however....... i have 4 nigh on brand new 3s 12v 3300mah 30c lipos sitting about (toy car is broke)

if i run them in series i get a tiny 3300mah 48v lipo. if i had that arranged with a relay/push button so i could momenterily attach the lipos to the SLAs in parrallel.

would it work as a boost button?


just throwing the idea out there....
let me know if it good or if im going to break something

thanks in advance
jim

 

[D8ve]

My first thought worst case is SLA at max charge circa 15 volts gives 60 volts at terminal. With 12 s lipo that's 5 volts a cell if you parallel them up. That could be press button and run time. A virtual bomb.

 

[mfj197]

Definitely not good I'm afraid, as D8ve says. Even if it did work all you would be doing is momentarily creating a larger capacity battery, so not increasing output power at all. There may be a possibility of wiring them all in parallel to give a 12V 9900Ah battery and have your button switching this in series with your SLA giving 60V instead of 48. That would give you a boost, assuming a) your motor and controller can handle it, and b) you can successfully switch in and out a battery in series at the high currents you are running!

Personally speaking I think you've got more mileage with your other thread, looking at converting to lithium chemistry overall and maybe increasing the voltage if the motor and controller can handle it.

Michael

 

[Deleted member 4366]

It would work as a booster battery if you put the lipos in parellel, and then put that parallel pack in series with your SLAs so that you have 48v + 12v = 60v. The main problem would be whether your controller can operate at the higher voltage. You'd lose the low voltage cut-off, so you'd need two voltmeters to monitor the two batteries so that you could stop when the voltage goes too low.

You need to check the main capacitor in your controller to see if it can handle (voltage marked on it) the maximum voltage, which eill be about 54v from the SLAs plus 14v from the lipos, which would be 68v. You might just get away with a 63v capacitor, but really you need 75v or more.

 

[jimmyhackers]

ahhhh i see.... thanks for the info. its helped my understanding loads

my idea bad, your idea good. its sparked my imagination, has anyone done something like this before? a turbo/boost button i mean

im gonna take my controller apart and do some component inspection soon. maybe replace things if needed.

sorry more questions now

at the moment i can get 30mph on a flat, 33 downhill and 25 uphill. the motor doesnt lag at all, no heat.
if i was to up my controller/motor to 60v and they could take it. what top speeds would everyone estimate i could get in each condition?

bit of a maths/theoretical one here. my motor and controller are 48v and 1000w so with P= V x I i get a max I of abut 21amps. if i then switch over to 60v will my controller still run at 21 amps (1250 watts) total. or will it tend to draw a higher current also? making for a higher wattage? if so, can this be estimated?

last question kinda combining the last two sections. i get generally amps = torque, voltage = speed, but i want to know something more specific.
at the moment at 48v i get from 0 to 30mph in about 7ish seconds. at 60v will this 0-30 time get better, worse or stay the same?

if all is good its a simple yet time consuming case of 2 relays a button and wiring. i have a tiny lipo voltage monitor so thats covered.

i cant wait, who wouldnt want a boost button

thanks again for the help

jim

 

[Deleted member 4366]

At the same voltage, torque is proportional to current, but at higher voltage, you get more torque too as well as power. The controller limits current, so you get the same current regardless of voltage. If you change from 48v to 60v you'll accelerate faster, reach a higher top speed and climb better. The only downside is that your high efficiency will happen at a higher speed, so your motor will run hotter at low speeds when you use full throttle.

 

[jimmyhackers]

thanks for the info. very useful

i will be able to negate the low rpm causing heat by using 48v at slower speeds.

 

[trex]

48V at low speed on a direct drive? - not a good idea.
At relatively low speed (under 200 RPM for 48V DD motors), the motor efficiency is below 50%, most of your battery power is going to turn into heat.
DD motors shine only when you can push 20A or more into the copper.

 

[jimmyhackers]

200rpm on a 26" wheel makes for....roughly 15mph.
im never really below 20mph besides from taking off. so alls well