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I have a slight doubt here. Can the outputs of 2 boost converters set to output the "same" voltage (isolated, they would never be exactly the same obviously) be safely tied together, as long as there is a capacitive load? If so, is there a minimum to that load?

Context:

schematic

simulate this circuit – Schematic created using CircuitLab

I need at least 4Ah of battery capacity, and I was only able to find single cells charger+boost modules, so I have to put them in parallel. It's for long term logging, the current is very low: 30mA peak, and around 5mA 99% of the time.

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  • \$\begingroup\$ Why would capacitive load matter? \$\endgroup\$ – Adam Lawrence Nov 17 '14 at 17:18
  • \$\begingroup\$ I didn't think it through, and thought at the time that the capacitor would present some inertia and the voltage fluctuations (times current) would be dissipated in the transistors of the converters. I'm really not sure anymore... \$\endgroup\$ – Mister Mystère Nov 17 '14 at 19:45
  • \$\begingroup\$ I wouldn't trust this module - it looks to me like they can't even get the schematic right - nowhere it seems does GND1 connect to GND on the switcher. Maybe I'm missing something? \$\endgroup\$ – Andy aka Nov 17 '14 at 19:54
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Given the diagram you have drawn in your question I would be tempted to look for two buck converters that each produce 2.5 volts on their outputs and then wire the two converters in series to give you 5 volts. You say they can be ran "isolated" and therefore this should work. When it comes to charging them, two charging power sources will be needed and these also need to be isolated.

Alternatively, find a charge-control circuit that is suitable for 2 x LiPo in series and then use a buck regulator to down size the raw output to 5 V.

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  • \$\begingroup\$ That's an excellent idea, I didn't think about summing the two voltages. And actually I wonder if it's not better regarding the discharge balance as well. \$\endgroup\$ – Mister Mystère Nov 18 '14 at 0:39
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I can not tell if you can put two of these modules in parallel without seeing a circuit diagram.

However if one module can deliver enough current for your application, you can do it with only one module.

LiPo cells are a bit picky when it comes to voltages above 4.2V. When you put two cells in serie the maximum charching voltage would be 8.4V. Now consider one of these cells is defective and ceases to charge at 3V, the other cell will now get 5.4V - too much. Thats the reason why in a serial configuration each cell needs it's own charging or balancing circuit.

The key here is that you like to put the two cells in parallel, they will be charged up to 4.2V, no problem. They will not catch fire or explode because of over voltage.

As usual there are drawbacks, in this case two.

  • The charging current will be split, the charging time doubles.
  • If one cell is defective it can discharge or even demage the second one, they are not isolated from each other.

As a sidemark, before putting them in parallel I would bring them to the same state, e.g. fully charge them separately.

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  • \$\begingroup\$ Thanks for the answer. I've updated my post with a bit of schematic and a current figure (5mA 30mA peak). The thing is, the circuit must be inherently safe, I can't risk putting batteries in parallel and hoping that it works. I could potentially put schottkies in series but I'm trying to avoid voltage drops. Is it the only option? \$\endgroup\$ – Mister Mystère Nov 17 '14 at 19:59
  • \$\begingroup\$ I was bit unclear about the circuit diagram, it should be from the charger/booster - sorry. As i've already written, putting two cells in parallel is not unsafe, that is often done in commercial products. By the way, can't you find a larger usable cell? The price for a second charger could be high enough to buy a new cell instead. Additionally when you put diodes in series, how will you charge the cell(s)? \$\endgroup\$ – Kitana Nov 17 '14 at 20:20
  • \$\begingroup\$ It's difficult to find single cells past 2Ah, or at an unreasonable price. I did just found a 4500mAh though, but I'm still interested in the answer for future projects. Regarding the diodes, I meant in series with the output. The charging circuit will be unchanged. \$\endgroup\$ – Mister Mystère Nov 18 '14 at 0:37
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I know this is an older post, but I wanted to mention that parallel sources like this do not inherently split the load current very well. Supplies are often paralleled in commercial power systems but they have circuitry built in to evenly split the current demand between all supplies. There is often an extra connection between each supply that allows the supplies to match voltages very accurately and thus supply currently evenly. It's true you can do OR-ing diodes, as they are called (they work as a logical OR function) but as you already said, this causes power loss, and it still does not result in real sharing. Sometimes this is done for simplicity in redundant systems where either supply can handle the load independently, in the case that one fails.

At minimum, OR-ing diodes are a good idea, but if you consider the topology of a boost converter, it has a diode inherently in the design. I don't think you would get reverse currents when one goes dead. If you aren't bothered by the fact that one battery would likely go dead before the other, then maybe this scheme could work. There could be efficiency tradeoffs in some ways also. I think you will get overall worse efficiency at low load conditions, because you will have two converters switching even if only one is needed. There are losses in switching converters any time they are switching the inductor.

One last note for the future, 3.3V is really suited well to running on Li-Po and a lot of devices are coming with Li-Po charging circuits built right in :).

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  • \$\begingroup\$ "they have circuitry built in to evenly split the current demand between all supplies." Is there a name for this type of circuit? \$\endgroup\$ – Skaevola Oct 13 '16 at 15:55
  • \$\begingroup\$ Yes, it's simply called current share/sharing, or sometimes current balancing. There are some papers on the web about methods and also it's possible to add on to converters if they have any kind of voltage compensation accessible. \$\endgroup\$ – John Lewis Oct 14 '16 at 17:13
  • \$\begingroup\$ When you say a lot of devices come with lipo charging circuits, what sort of devices did you mean? MCUs? Boost regulators? \$\endgroup\$ – CL22 Nov 5 '16 at 17:13
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Made and test this yesterday with two of these very excellent boards: https://nl.aliexpress.com/item/wholesale-2-pcs-lot-DC-DC-2A-Adjustable-Step-Up-Boost-Power-Supply-Converter-Module-2/32266587114.html

To power an Amp (2x50W, class D, very efficient) with 3.7V 8000mah (4x2000mah) battery to 24V (4A) and it's working great! Really.

Device I made to power an 24V Amp

On each board there is a 3A schottky diode present that protects the output so it is safe to connect these boards together (diode is not shown on the aliexpress product picture, dunno why).

For best results you NEED to be SURE both modules are connected to the same source (and switched on at the same time) and output EXACTLY the SAME voltage. In this case it is 23.8V and I use a large 25V capacitor 4700uF connected to both output 'lines'. Before you connect these modules to eachother, you have to setup first the voltage! I made a jumper on the back to be able to disconnect or connect the boards to eachother to be able to setup or to troubleshoot.

Tested the boards in this configuration and measure the voltage at high load and it seems to be pretty stable (almost no voltage drops until you reach INSANE because the Amp is able to output 100W and this config can't reach this). Very reliable, very nice. I will try to connect another board to get at least 5A so it can handle the full potentional of the Amp.

So yes, it is possible.

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