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I would like to design a circuit that charges 3 Li-Ion batteries in parallel with a 1S charger, then switches the configuration to put them in series to discharge as one 11.1V battery pack. This is quite similar to the design in this answer to a previous post: https://electronics.stackexchange.com/a/219619/152168 (except I'm using 3 cells to power a Jetson TK1, and my switches are a series of relays).

This will be used as a backup battery for a 12V main supply that may occasionally cut out. This is one of my design constraints, I cannot use a different source as my main power. Aside from that, if you have any suggestions for design changes, I'd be happy to hear them.

My first question is whether or not the batteries will fall out of balance when they are discharged is series. The aforementioned old post made no mention to whether or not balance was an issue when switching between configurations. If they do become unbalanced, then I'll have to figure out a separate balance circuit (which I'd prefer to avoid). The cells are all from the same shipment, which I've been told will help them stay balanced. Also, the batteries will never be relied on for longer than 30 minutes at a time, so I will not likely be running them close to low charge.

On another note, I've found that when switching power sources with relays, the Jetson momentarily loses power and shuts down. I plan to avoid that by using some sort of capacitor to temporarily power the Jetson in that moment while the relays are switching contacts. I have very little practical experience with capacitors, so I'm not sure how I would integrate this into the circuit. Could you explain (or show) where the capacitor would go in relation to the batteries, relays, and load? You can refer to the schematic below for an idea of what I want to do. Sorry that it's so messy, I'm new to drawing circuits.

schematic

simulate this circuit – Schematic created using CircuitLab

I also don't know what capacitance I will need to keep it running during the switch. I understand the C=Idt/V formula, but I'm not sure how to tell how long is needed. If you wanted to do the calculations yourself, the accepted voltage drop is 2V and the current is about 1A, so C=0.5*dt. How do I determine dt given a particular relay?

If there is a better/easier alternative to relays or any other part of my design, please let me know.

Thank you

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  • \$\begingroup\$ "My first question is whether or not the batteries will fall out of balance when they are discharged is series" - yes! If you want to run Li-ion cells in series, you need a BMS of some sort. Your relay arrangement alone won't do. \$\endgroup\$ – Dmitry Grigoryev Jun 30 '17 at 8:21
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No offense, but what you propose is a kludge solution. There is a strong possibility of a catastrophic failure due to relay timing issues and failing to do proper charge management.

There are battery charging ICs that can charge these cells in series, take care of charge balancing, and no need to do any switching to draw current from the battery. As one example of a very basic chip, take a look at the LTC4079. There are many such chips on the market.

Here is a simplified circuit for how to use a chip like the LTC4079:

schematic

simulate this circuit – Schematic created using CircuitLab

Note that the LTC4079 does not include cell balancing. Most cell balancing ICs require microprocessor control, at least to set it up initially. Take a look at the ISL94203 chip as an example of one that you can configure after which it will run autonomously.

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  • \$\begingroup\$ Thanks for the tip. I looked at the data sheet for the LTC4079 and it should work to charge my battery, but it was unclear to me whether or not the chip also handles power path management when the main power is cut. If it does, could you explain where the load output comes from on the board? (link to the data sheet: cds.linear.com/docs/en/datasheet/4079f.pdf) If it does not, what should I use as a switch between the two sources? And I suppose I will need a separate BMS to keep them balanced while they're charging? \$\endgroup\$ – RaptorT Jun 30 '17 at 14:59
  • \$\begingroup\$ If you can tolerate a small voltage drop (~0.2 volts) then a Schottky diode in series with each source is a nice solution. \$\endgroup\$ – Glenn W9IQ Jun 30 '17 at 15:51
  • \$\begingroup\$ If you want BMS as well, you can find a chip with this all integrated \$\endgroup\$ – Glenn W9IQ Jun 30 '17 at 15:55
  • \$\begingroup\$ I have been researching schottky diodes, but I have not figured out how I would use one here and what it would do. By "in series with each source," do you mean that each source is connected to the load separately with a diode in series with it? Do you mean the batteries are in series with each other with a diode in between? Is the backup being simultaneously charged and discharged so that when the main is cut, it does not have to switch sources, it just draws from the battery alone? Sorry, I'm new to this and I'm learning as I go. \$\endgroup\$ – RaptorT Jun 30 '17 at 19:10
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    \$\begingroup\$ I have been incapable of finding a chip that handles charging, balancing, discharging, and power management all in one place for a 3S battery. If you know of one, could you please point me in the right direction, or at least tell me what to search for. \$\endgroup\$ – RaptorT Jun 30 '17 at 19:13

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