Hot-swap battery power system with in-situ charging

Thread Starter

nopaddle

Joined Dec 5, 2022
4
I'm trying to design a power system for a UAV (drone) Ground Control Station (GCS). This is essentially a couple of high-brightness displays, a computer, a switched mode power supply, DC-DC convertors and a few batteries in flight case.

We need the system to have hot-swap batteries, and for it to run and allow the batteries to charge while connected to shore power. I've prepared a block diagram for the power system, as shown below.

1670415937447.png

  1. I'll also add fuses where appropriate.
  2. These are the ideal diodes that I'm planning to use.
  3. For the batteries, I'm looking at ebike batteries, batteries.

Does it look like this will work, or have I got part of it, or all of it wrong?

There is another post on here about charging multiple batteries from one charger that I found useful.
 

LowQCab

Joined Nov 6, 2012
2,627
I don't see why You would need 8 "Ideal-Diodes".
As long as the Meanwell-Power-Supply is set up correctly,
You should be able to have any number of Batteries connected, including no Batteries at all,
with the Buck-Converter treated as "just another Battery".

The biggest problem is,
balancing the Charge between the individual Cells of each Battery,
as they are being Charged.
This is vital to the expected Life-Span of the Batteries.

There is an additional problem of the Current-Limit setting of the Power-Supply.
The Current must be limited to the maximum Charge-Rate allowed for an individual, single, Battery.

This means that when 4-Batteries are connected,
the Current will continue to be limited to the maximum allowable Current for one Battery,
and the Power-Supply will reduce it's Output-Voltage to limit the maximum-Current.
This might still work since You have a higher than needed Voltage feeding a Buck-Converter.

So, if You plug-in 4 dead Batteries,
they will all be charged at 1/4 of their maximum allowable Rate.
And it won't be evenly split between the Batteries.

You will need 4 individual Power-Supplies, and 4 Ideal-Diodes, and
a large Bulk-Storage-Capacitor on the Input of the Buck-Converter, ( maybe around ~10,000uf ).

There is no getting around it,
if You want to charge 4 Batteries at the same time,
You will have to have 4 Battery-Chargers, ( each with Cell-Balancing-Circuitry ).

When set up in this manner,
You can add or remove any number of Batteries whenever You like.

I would suggest a proper Battery-Charger
which is specifically designed to handle 4 (or more) Batteries simultaneously,
( which will also include Cell-Balancing-Plugs for each Battery ),
but 4 smaller Chargers will also work.
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Thread Starter

nopaddle

Joined Dec 5, 2022
4
Thanks for the comment. To start with, I just had four ideal diodes, but then needed to link them all on the upstream side of the diodes to get the charge current to all the batteries. I then realised that would negate the whole point of the diode, as any in-rush current when hot-swapping could just by-pass the diodes. - So I added four more.

I was hoping the BMS would protect the batteries from charging too fast, without limiting the charger current, then the current supplied to the load would not be limited by the battery charging limits. Have I got that wrong?

It's a good idea to use a charger with multiple outputs, Mean Well don't seem to have anything suitable, but I will look elsewhere. I will still have the problem that the load might need 600W, or 12.5A and I the maximum charge current summed across all the batteries is quite likely to be less than that.

I'm a mechanical engineer, so am a bit out of my comfort zone and am intersted to know why my eight ideal diode solution won't work (assuming the BMS can handle the current limit).
 

LowQCab

Joined Nov 6, 2012
2,627
A well designed BMS on each Battery could change the plan,
but I know nothing about your Battery's BMS Circuitry, or lack thereof.

I have to assume that You have bare Batteries until more is known about them.

They could have Charging-Control-Features,
or maybe just Under-Voltage-Protection.

If the Battery's Specification-Sheet doesn't specifically address Charging procedures,
then it's a safe bet that they don't offer any Charging-Control-Features on the integral BMS-Board.

I would be surprised to learn that the BMS-Board
automatically controls Charging-Current and Voltage and Cell-Balancing.
These factors are very important for safety, and Battery longevity.
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Thread Starter

nopaddle

Joined Dec 5, 2022
4
We have used LiFePO4 batteries from K2 Energy before, and they can be handled like a lead acid. We just had one battery to worry about and had it floating on the output from a AC-DC convertor.

I can't find a lithium ion with an equivalent BMS, and no enough about lithium batteries to treat them with a bit of respect.

With this in mind, I've taken your advice and attempted a wiring arrangement with a charger for each battery, and a separate branch to provide enough power to run the system while on shore power, while respecting the battery charging limit. How does this look?

1670595996581.png
 

LowQCab

Joined Nov 6, 2012
2,627
Without a Schematic of the 2 SMPSs,
it's hard to say if either one of them can tolerate having Voltage applied to it's Output
when the Input-Power is switched off.

If You can supply Schematic-Diagrams for both of them,
it might be possible to make a judgement call on whether this could be a potential problem.

On the other hand, You can just install an Ideal-Diode on the output of each SMPS,
and then You shouldn't have any issues.

Another option would be to put the Switches on the Outputs of the 2 SMPSs,
and remove the Switches on the Inputs.
( the idling-Current should be fairly low,
but this arrangement will eventually drain any
connected Batteries if there is no AC-Power to the Chargers,
so for that reason, I wouldn't recommend this setup ).
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Thread Starter

nopaddle

Joined Dec 5, 2022
4
Good point about adding ideal diodes. I've not finalised on the SMPS yet so will assume the addition of ideal diodes on the output. Many thanks for your help.
 
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