# Multiple batteries vs one high voltage battery + stepdowns

We are building a system which is composed of modules that have different power demands. The system should be powered by Li-ion batteries, which have to be monitored and protected (overvoltage / undervoltage / overcurrent...).

For example:

1. Control unit — ~5 V, ~1 A
2. Servo unit — ~7.4 V, ~10 A
3. Ignition — ~11 V, ~20 A
4. DC Motors — ~14.8 V, ~20 A

Now we thought of two approaches to meeting those demands.

Big Battery pack + Buck converters

Because, due to balancing issues, it is obviously not advisable to directly tap those different voltages from matching cells in a serial battery configuration, step-down converters have to be used to produce adequate voltages.

With large powers, the efficiency of such converters comes into play.

At 95% and 595 W peak power demand, we would lose around 31 watts on those regulators.

In relation to the 595 watts this is of course a small fraction.

Nevertheless, my friend proposed the following, and I am curious:

Multiple smaller battery packs

For ignition and every motor, there can be a dedicated power source, loosely matching the required voltage. The control unit would use an additional LDO regulator in order to adjust its voltage more precisely.

This would reduce the amount of voltage regulators, but we would need to monitor every battery pack separately. (A protection and monitoring/balancing circuit per battery pack.):

simulate this circuit – Schematic created using CircuitLab

Also, my guess is that large currents across different sections of the common ground would couple the different modules with each other in a unwanted way. This could lead to damage in more sensitive parts of the circuit and affect analog controls.

Therefore, it might be wise to isolate the modules and optically couple necessary signals, but this adds additional complexity.

Questions:

Which architectures should be used (would you use) depending on what situations?

1. Big battery + buck converters
2. Multiple batteries
3. Multiple batteries, galvanically isolated modules.

What to consider when routing a common ground where currents from multiple power sources meet?

Is balancing an overkill when using in a system that is only able to discharge the battery?

Monitoring & protection circuitry can't be affected when combining multiple grounds, is that correct?

• If the system is operated from just one battery, the time it can be used does not directy depend on the most used kind of load. Is this relevant?
– Jens
Commented Aug 1 at 21:08
• Not very much to our scenario, but generally yes. In a multi-battery configuration the lifetime depends on the shortest-living module-battery pair. Charge can't be moved between batteries. In an single-battery configuration on the other hand it might be a little bit more difficult to keep track of the power usage of different modules. Commented Aug 1 at 21:15
• The stored energy will be almost impossible to balance for every scenario such that only < 5% of the total initial energy remains. Since the motors use almost half of the total, can these be matched a single pack to avoid the regulator loss but maybe have excess power available initially? Look at how all the voltages are created in a forward converter for a PC PSU all regulated from the main output and tightly coupled in one transformer. This is an alternative strategy. Look at the 1kW version schematics. You may also compare boost forward converters from fewer cells with BMS. Commented Aug 1 at 23:28
• Is total mass also important for excess energy density with multiple packs? Then for drones excess weight has an added efficiency loss. Commented Aug 1 at 23:28

If you want complex then use one of the multiple battery configurations, this would be more costly in time and probably BOM cost to implement.

Galvanic isolation could allow you to decouple the batteries from the loads and do things like add voltages together and put the DC/DC's in parallel (which presents a host of challanges)

If you have a choice and aren't requirement driven to use one of the other typologies, go with simple and use one battery and one DC/DC converter.

Definitely:

1: Big battery + buck converters

A 12 V battery can power your high power loads directly (Ignition and DC motors)

That way only the lower power loads require buck regulators. That's the most power-efficient solution and the cheapest one.

Use a single big battery.

If possible re-design some of the loads to be tolerant of the battery voltage range, else you'll be needing DC-DC converters for all the loads.

Using multiple batteries would probably mean that you end up with one of the batteries depleted but all the others with still untapped potential. So even though it may be more efficient on paper it hasn't given you a longer run-time.