# What are ways of isolating/splitting one DC power source

I'm building a 6S Lithium Ion battery pack with a nominal voltage of 22.2v. The pack will be enclosed so taking the batteries out to charge them is not practical; therefore, I'll need to add the charging circuit to the pack. Ideally, I'd have 6 TP4056 modules charging each li-ion cell and wired like this:

However, that is not possible with these modules as IN- and B- shorted together by design. Wiring it like that would short the batteries.

To avoid this, I'd have to have isolated power supplies for each module. 6 individual wall wart 5v power supplies would do the trick, but that is clumsy.

Ideally, then, I want to power 6 TP4056 boards with one 5v source.

• Is it possible to "split" that 5v power source into 6 isolated 5v sources? I read somewhere that capacitors can be made to isolate DC sources, but I don't see any sources on the web backing that up.

• Is there any better way to retain a 6S configuration yet be able to safely charge each cell? Obviously there are BMS's, but I do not have any stores that cell them near here. There are, of course, these ebay things but they do not have any datasheets.

6 DC-DC converters are not an option for me. I have literally scoured the internet today and am tearing my hair out over this issue. Would diodes work anywhere?

• Here's a sample schematic, except that they've labeled your $P_+$ and $P_-$ as $OUT_+$ and $OUT_-$: module schematic. It's easy to see that there are only four distinct nodes here, as $OUT_+$ and $B_+$ are tied together and so are $IN_-$ and $OUT_-$. So you have correctly recognized that these will not work as you would like. I don't like the idea of using these things in series, regardless, as one might be "weak." But other than adding switches to flip from parallel to serial, I'm not sure what to suggest here.
– jonk
Jan 21 '18 at 6:13
• What capacity are the cells, and how fast do you want to charge them? Jan 21 '18 at 7:25
• @BruceAbbott Each cell is rated for 2600mAh but are taken out from an old laptop battery. IIRC the wear level reported by the SMBus was 80% so I'm assuming 2000mAh. I'm not worried about the charging speed, overnight charging (C/10, etc) is fine for me. Jan 22 '18 at 9:17
• @jonk I'm not bothering with switches anymore. If I used switches, they would have to be ganged else bad things will happen. The only switches I have are SPDT ones bought off eBay. If I was going to use switches, it would have to be a "Break before make" connection. Thanks for your input. Jan 22 '18 at 9:18

Yes, there are modules to convert DC to DC and both sides are isolated. But it is relatively expensive and current capability is not high.
I wouldn't recommend this to your problem. I would recommend you to use proper 6S charging module with balancing.

• I searched online and it seems that the B0505S-1W is a chip that takes and input and isolates it. Fortunately, it has a datasheet here: google.com.au/… Unfortunately, the output power is mediocre 1W, whereas the TP4056 modules typically require 5W (5v @ 1A). I am not sure if it will overload the B0505S and cause issues. Do you have any idea if it limits current by design? Jan 22 '18 at 9:26
• To be clear, I really don't care about efficiency whilst charging though. As for the BMS idea, do you have any idea how to charge batteries using them? Would I simply give a 6S BMS a 4.2v*6=25.2v input? I have asked the eBay sellers for datasheets and clarification, but I doubt I'd get a linguistically/technically correct response. Jan 22 '18 at 9:31
• @Che0063 it is not supposed to be used for charging, you can draw 200mA (max). To be clear, I do not recommend this in my answer. BMS is a way to go. Of course do not expect any sort of documentation from chinese sellers. Documentation for BMS should tell you what voltages can be on the input. Of course it has to be larger voltage than 6*4.2V Jan 22 '18 at 9:47

To operate multiple TP4056 modules in series you need isolated power supplies. This can be done using a transformer with multiple output windings, but it would probably have to be custom made. Some commercial chargers use this technique, however complexity increases with the number of cells so it is generally limited to 2-4S chargers.

The usual method is to charge all the cells in series with a single CVCC (Constant Voltage Constant Current) regulator, while using a 'balancer' to equalize the voltage on each cell. Balancer modules often also include protection against over-voltage, over-current, and over-discharge, and some can take an optional temperature sensor. These protection features are well worth having, particularly when using old cells which may have different capacities and internal resistances.

The balancer module can be installed in the battery, with the charger connected through the positive and negative battery wires. The charger is then powered from a voltage source a bit higher than the battery. With this setup the device can be operated from the mains, with the DC/DC converter providing power to the device and charging the battery at the same time.

Cheap CVCC DC/DC converter modules and 6S balance/protection boards are readily available on eBay and elsewhere. You will also need a power supply that outputs ~28-30V, or alternatively use a boost converter to raise the voltage (but remember that it must draw more current to get the required Watts at the lower voltage).

Older li-ion cells have a maximum charge voltage of 4.1V, so for safety you should set the DC/DC converter's output voltage to 4.1x6 = 24.6V or less (a lower voltage will also maximize whatever lifetime the cells have left). Set the charging current as low as you can while still providing acceptable charge time.

Before making up the battery pack you should test each cell by charging and discharging it individually. Then after the pack is made up you can charge each cell individually again to ensure that they all have equal voltages at full charge.