I’m making a small tracked vehicle that weighs nearly 1kg and I'm going to use two 3.7v 1200 mAh li-ion batteries but as a newbie I'm not quite sure if that's enough. The components that draw the most power are the two 6v 48:1 dc gear motors and a SG90 servo motor, the rest is pretty insignificant. With two 3.7v 1200 mAh batteries I would have 17.76 Wh. According to this datasheet the gear motors draw 2.34 W at 4.5v when stalled and the servo which has an very light load typically draws 0.8 W. That should mean that I can run the motors continously even when the dc motors are stalled for 3 hours unless I got something wrong. I need the batteries to survive a 2 hour fair where the tank won't be running all the time so that seems to be more than enough but if someone has made a similar project it would be nice of them to confirm.
Why not just test it and then you will know how long the batteries will last?
I'd do that but I don't have the batteries yet.
But do you have the chassis?
Then how do you know the whole machine will operate?
Battery capacity claims tend to be optimistic (sometimes outright lies). Also they measure to an voltage endpoint that is just above failure - which may be below a usable level for a given project.
Why yes, I have had problems with battery power - even after doubling what the specs claimed.
I don't know for sure, I thought that I should find how much capacity I need before I buy the batteries.
What are You thinking about? A DC motor being stalled will quickly overheat. Hope it's just a language mistake.
Agreed. Not to mention amp hour capacity vs watt hour capacity and how misleading those can be used.
I think they're just referencing max draw from the manufacturers spec sheet but don't intend to test or use it that way. I hope.
I'm not going to stall them. I meant that they can run for 3 hours at maximum power consumption.
Isn't it volts * amp hours? That's only 4.44 and 8.88 for two batteries. Plus you're using 3.7 which the batteries may not sustain that voltage the entire time. The overall average may be lower than 3.7 further reducing capacity.
Or did I math wrong?
You're right. My mistake was calculating 7.4 * 2.4 instead of 3.7 * 1.2 * 2. Apparently the charging cut-off voltage of a 3.7v li-ion is 4.2v and it's discharge cut-off voltage is 3v and the average of those is 3.6 so that makes 8.64 Wh. On paper that should still be enough but I think will use two 1500 mAh batteries for 10.8 Wh just to be safe.
Ah, Ampere Hour.
Using the nominal V's of a LiFeP04 at 12V and a battery rated at 100Ah and takin into account that LiFePo4's settle down to 80%/85% efficiency. It means I should be able to count on a 100Ah LiFePo4 to run for 1 hour at a load of 80A. If the load was 40Amp, I'd expect 2 hours of operation, 20 amps 4 hours of operation.
Li-ion not LiFeP04, but it doesn't change your point does it?
The point is not changed.
Stall current applies to the first milliseconds of the motor start. That's data needed for the driver. For battery capacity calculations usually the steady running current is the best to use.
Make the best assumption You can regarding battery capacity, run the project measuring current consumption and battery voltage. It depends a lot on the duty cycle of the activities.
I'm aware, but thanks.
Good. Take a look at the over all running, the duty cycle of the lot. 10 seconds every minute, every hour, continues running.... That's from where battery capacity is calculated.
I somehow got the calculations really wrong. The motors draw 5.28 W when stalling instead of 2.34 and 2 x 1200 mAh batteries have 8.64 W but
So that's more like 6.48 W which means 30 minutes at an unrealistic power consumption rate. 45 minutes with 2 1500 mAh batteries. That should realistically be more than enough, right?
Hi @Railroader , you're replying to me but I'm not the OP.
Regards.