Tapping different voltages from a single battery pack?

Mainly asking out of curiosity here. Let's say I have a 48v 14s 20ah li-ion battery. Battery has standard cheap chinese 14s balance circuit installed.

Under those circumstances, is it possible to tap a lower voltage from the battery to use for something with a small load? For example, if I needed 14.8v, and I only tapped off of 4 cells, leaving the balance circuit installed normally and 48v still available. Would the balance circuit be able to make up for that if it was only a very small draw, say less than an amp?

Use a bus bar, then it dosen't matter the load as it will pull what it needs.

Tony Scott said:

Use a bus bar, then it dosen't matter the load as it will pull what it needs.

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Huh? I have no idea what you are talking about. I'm talking about pulling two completely different voltages from the battery, using 4s and 14s simultaneously.

There is no question doing that would cause a battery to become unbalanced, but can a small balancer make up for that?

I wouldn't do it...
It would probably make your cells unbalanced. Most BMS only handle a few tens or maybe a few hundred mA balance capability.
You should use a DC-DC step-down (buck) converter. There are some that are adjustable, so you can play with the output voltage.

I have this one for example:
Which is nice because you can set the output voltage and amperage (and other various protection paramters, such as low-voltage cutoff etc...)
But, I'm not sure it will suit your case, because the input voltage is limited to 50v max. So if your 48v nominal pack is above 50v when fully charged, this specific model might not be enough.
But i'm sure there are other models that will be able to handle a fully charged 48v pack.

I have a 24v pack, but needed to run some 12v loads. I used a dc-dc voltage converter off the main output to do this so I don’t off balance the cells. If the load was some LEDs or something and the bms could compensate I might be tempted, but the converters are cheap and then no worries about charging or discharging unevenly.

I just installed a converter for a 24 volt pack also. Although its not 100% efficient, it way outweighs pulling unevenly. For me usually its a negligible 12 volt draw, but there are days when I need to turn the propane heater on and that 12 volt blower motor fan pulls quite a bit of energy.

As long as the balancer can transfer enough power to the lower 4 cell’s you’re good.

The 14.8 V part must draw less amp’s then the balancer max amps.

Number21 said:

Huh? I have no idea what you are talking about. I'm talking about pulling two completely different voltages from the battery, using 4s and 14s simultaneously.

There is no question doing that would cause a battery to become unbalanced, but can a small balancer make up for that?

Click to expand...

DC-DC converter are pretty efficients.
I use one to light my LED strip from a 48V pack (48v->24V), when i push the button (48V button), it switch the dc-dc that power the leds, this way i got 0mA wasted when lights are off.

Honuz said:

DC-DC converter are pretty efficients.
I use one to light my LED strip from a 48V pack (48v->24V), when i push the button (48V button), it switch the dc-dc that power the leds, this way i got 0mA wasted when lights are off.

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A good way to do it for a single item draw. I have two breaker/ fuse panels for my system, one is standard 120 vac, other is 12 vdc and 24 vdc for equipment that runs on that. Because it’s a buss bar with multiple outputs I can’t really Switch it off.

Number21 said:

Mainly asking out of curiosity here. Let's say I have a 48v 14s 20ah li-ion battery. Battery has standard cheap chinese 14s balance circuit installed.

Under those circumstances, is it possible to tap a lower voltage from the battery to use for something with a small load? For example, if I needed 14.8v, and I only tapped off of 4 cells, leaving the balance circuit installed normally and 48v still available. Would the balance circuit be able to make up for that if it was only a very small draw, say less than an amp?

Click to expand...

Using 3 DPDT switches you could "tap" off of 4 sets of 4 cells on your 48v bank. Cycling through each set of 4 would reduce the balancing required.

One amp is huge. Get it below 50 milliamps and the effect will be minimized. Regardless... I don't recommend it.