Need help from BMS experts. I have an unusual problem, unusual in the sense of esk8 application and pretty usual in terms of how laptops charge. The problem at hand is to have a BMS+battery setup that can power my computer (max 30 Amps usage) while simultaneously charging (@ 10 amps max) the battery. Are there specific chargers I can use? Specific BMS or circuits?
Background: I work on small ground vehicles (robots) and have been using separate battery for computer and the drive system. The batteries have their charge and balance port exposed so they are charged using hobby chargers. But I am working on a new robot and I decided to have a BMS with just a charge port.
Yes, obviously, and that’s what I currently have. But the issue is when I want to use the computer and charge at the same time. Just like a laptop. So now the charger has to be capable of supplying 40 Amps (10 for charging and 30Amps for computer). But if I now connect that 40 Amp capable charger to BMS, it’s going to charge at 40Amps, which for sure will destroy the battery.
You should use a dedicated charger for your battery, and a separate power supply for the laptop. You could use a CCCV DC-DC converter to charge the battery from main power rail. And then another DC-DC to get a suitable voltage for your computer.
This. You really need to use two separate power supplies.
Laptop chargers are designed to supply constant voltage while battery chargers are designed to supply constant current.
Any solution that allows you to use one charger for both is going to require many additional components and will not be worth the trouble.
Well I don’t have a laptop. I have two computers and several sensors powered from this main battery. I want to run the computers while charging them. Just as one does with a laptop.
That doesn’t sound like a polished solution for me. Might as well go back to the hobby charger. Also if laptops are able to charge while being used then it’s a solved problem. There should be circuits or BMS out there. I am not too concerned about the cost.
You are able to power your computer off your battery, and the computer can draw upwards of 30A from the battery.
You also want to charge the battery at 10A, but you also want the charger to be able to supply power to the computer if the computer is in use, correct?
Charger -> battery -> computer.
When computer is on, charger supplies 40A, when computer is off, charger supplies 10A?
When I connect the charger, the battery should be charged at max 10A and the charger should also be capable for providing upto 30Amps requuired for runnign the computer.
Ok then we’re going to need something more then.
The issue is that the computer is not in between the charger and the battery. There is no way for the charger to detect that the computer is on and drawing power.
I don’t see a solution that is not going to require multiple connectors and additional components.
Basically you’d need a super smart BMS that has a separate discharge path AND can regulate the charging current by use of an internal DC-DC converter.
Then you’d need source selection circuitry for the computer to either draw power directly from the charger or from battery discharge path.
It’s a pretty specific use case, you may end up needing to design a custom BMS to accomplish this.
The BMS needs the following features:
Internal DC-DC converter to regulate charging current
Output source selection switch that switches between battery terminals or charger input, depending on whether the charger is present that switches in a break-before-make fashion.
It’s like a USB battery bank controller on steroids.
I’m thinking that the solution is fairly simple…
Assume the pack and BMS are one entity, never apart. If charger and pack outputs are paralleled with no load then the pack and charger operate normally and the pack is charged at up to 10A.
If the outputs of pack+BMS and the charger are then connected to a load drawing 30A then the pack+BMS and the charger will divide that current based on their internal resistance. I suspect that the charger will supply its full 10A, leaving the pack to supply the other 20A. The charger will never charge the battery when more than 10A is drawn by the load.
When the load is off then the battery will supply zero current to the load but the charger will still be providing 10A. This is used to charge the battery now though.
As long as the pack and BMS are kept “tight” together as a single unit then I don’t see any issues with just paralleling the charger and pack+BMS. The pack can never supply more than 30A (the max load), the charger can never supply more than 10A (its setting/rating) and the pack+BMS can never be charged at more than 10A.
I am NOT saying all this is definitely true! I’m only thinking out loud that this is a fairly simple situation with a simple solution. Don’t attempt to try any of this without testing with a much lower power setup first. All this, and any other actions you take, are at your own risk.
The thing is, he wants the charger to supply UP TO 40A when a load is connected, and 10A when a load is NOT connected, not just 10A all the time. So regardless of whether a load is connected, he wants the battery to charge at 10A when a charger is plugged in.
That’s where this gets complicated. He basically needs something like a USB power bank controller but for a 10S lithium ion battery.
The upside is that he wouldn’t need a CC-CV charger, as the controller would handle that part, and simply just a 42V 40A power supply.
