Tír Na HiFi

Hi Rick
Here are some suggestions:
You can individually charge two batteries in series by using a normal vreg to charge the battery on the ground end of the series & using an isolated DC-DC converter on the other battery in the series - the isolation avoids the input ground of the DC-DC converter being connected to the Pos+ pole of the other Vreg.

I use TDK-Lamda DC-DC isolators as they are fully regulated in 3.3V, 5V, 12V, etc outputs & come in a variety of power outputs 1.5W, 3W, 6W, etc & also come in various voltage input ranges 5V, 9V, etc.

The model range is CC1R5-, CC3-, CC6- - you'll find them in Mouser - for instance:
https://www.mouser.ie/ProductDetail/TDK ... 2bW4y14%3d

I've never liked the idea of running these LiFePo4 batteries at 2.5V * 2 to get 5V output - it's at the very end of their useful range - so you are not really using the battery as intended!

So here are some ideas:
- Use two supercapacitors @ 2.5V each (use the 2.7V supercaps to give some headroom) & charge each one individually using the scheme above (you can use a cheaper DC-DC converter than the TDK-lambda as your will be using a voltage regulator between DC-DC converter to bring the voltage down to 2.5V. The cheaper DC-DC isolated converters voltage output is not so well regulated as the TDK-lambda ones but that isn't of concern when using a TPS7A4700 to regulate the DC-DC voltage output down to 2.5V. Care has to be taken with this as the discharged supercaps will be seen a dead short to the voltage regulator.

- As above, two supercaps but using a balancing board which limits the voltage on each supercap to 2.5V or 2.7V & charge with 5V across the two supercaps via the balancing board. Care needed with Vreg, as above but you can use a way to gradually ramp up the charging current o avoid stressing the Vreg.

- Use a hybrid battery + supercapacitor: charge the battery to 3.3V & the supercap to 1.7V - again use of DC-DC isolated converter is needed & the same care needed. this way the battery is at it's full charge & supercap is fine at 1.7V - well away from it's 2.7V max range

Lots of variations possible.

tony wrote: ↑Mon Nov 26, 2018 12:15 am John i am happy to give them a go if there still available. I could try them on my ppang usb card maybe?

Georgi wrote: ↑Mon Nov 26, 2018 9:46 am I am looking for upgrading my PS on the miniPC, but not sure how that going to work.
It needs output 12V 3A.

Georgi wrote: ↑Tue Nov 27, 2018 12:27 am JKeny, did you get my PM?

Float charging is fine for some applications that is not that serious. However, to reach top level performance, float charging method has some limitations:

1. If the battery is not fully charged, it will behave as shunt element, the charger will play the most important role to the power supply.

2. If the battery is fully charged, it will work like a decoupling capacitor. The direction of current to the battery will be switched rapidly between forward and backward. Noise will be generated. Chemical reaction will be changed in between.

3. Power supply can not be isolated from the charger or other related circuit.

In either case, the battery power supply performance will be degraded. No longer as good as pure battery power. It will be the same story to the ultra capacitor power supply with float charge scheme.

jrling wrote: ↑Fri Dec 14, 2018 2:45 pm This was stated on DIYA re float charging A123 26650/Ultracaps in a PSU:

Float charging is fine for some applications that is not that serious. However, to reach top level performance, float charging method has some limitations:

1. If the battery is not fully charged, it will behave as shunt element, the charger will play the most important role to the power supply.

2. If the battery is fully charged, it will work like a decoupling capacitor. The direction of current to the battery will be switched rapidly between forward and backward. Noise will be generated. Chemical reaction will be changed in between.

3. Power supply can not be isolated from the charger or other related circuit.

In either case, the battery power supply performance will be degraded. No longer as good as pure battery power. It will be the same story to the ultra capacitor power supply with float charge scheme.

Personally, I have not found any deterioration with float charging or running batteries lower voltage than spec, but wondered if the assertions are valid and have merit?

Jonathan

jrling wrote: ↑Fri Dec 14, 2018 2:45 pm This was stated on DIYA re float charging A123 26650/Ultracaps in a PSU:

Float charging is fine for some applications that is not that serious. However, to reach top level performance, float charging method has some limitations:

1. If the battery is not fully charged, it will behave as shunt element, the charger will play the most important role to the power supply.

2. If the battery is fully charged, it will work like a decoupling capacitor. The direction of current to the battery will be switched rapidly between forward and backward. Noise will be generated. Chemical reaction will be changed in between.

3. Power supply can not be isolated from the charger or other related circuit.

In either case, the battery power supply performance will be degraded. No longer as good as pure battery power. It will be the same story to the ultra capacitor power supply with float charge scheme.

Personally, I have not found any deterioration with float charging or running batteries lower voltage than spec, but wondered if the assertions are valid and have merit?

Jonathan