3 ms·

On a tangent, but... do you think it's possible to build a battery that discharges as a pack (in series), but charges each cell individually? I imagine it will

by DenisM 6y ago

On a tangent, but... do you think it's possible to build a battery that discharges as a pack (in series), but charges each cell individually? I imagine it will require some creative wiring, but charging efficiency should then go through the roof, right? I image I went e-biking and returned to my car to recharge - having individually chargeable cell should reduce the overall charge time a lot.

PragmaticPulp 6y ago
A battery management system handles this. You still charge the pack in series, but the BMS will discharge cells that have a higher voltage than others. This results in all cells having similar charge levels. It doesn't change the charge time, though. You're putting the same amount of energy into the pack either way. 14.8V @ 100mA applied to 4 cells in series is the same energy as 3.7V @ 100mA applied to 4 cells individually. Can't escape the laws of physics. If power efficiency is critical (solar applications, for example), the BMS might have fancy switching circuitry to re-route energy from overcharged cells to undercharged cells. This is expensive, though, so most BMSs just use resistors to burn off extra charge from overcharged cells.
- DenisM 6y ago
  > You still charge the pack in series I understand that's how it's done, but can we charge faster if we DECIDE NOT to charge in series? I take it the answer is no? Imagine a series of 14 cells. I could charge them as a whole, but there is normally some limit on how much power I can pump into it. Now suppose that I disassembled the series and now I have 14 cells that I can charge independently before re-assembling them back. If I did that, would I gain anything? If nothing else this should prevent unbalanced charging, reducing overall heat of the pack and allowing higher current overall? I can see that some packs advertise 5a charge current and other packs of the same capacity offer 10a or even 15a. Wonder why is there such difference.
  - reportingsjr 6y ago
    It would be marginally faster (best case probably a percent or two) for a much more expensive charging circuit. The reason for this is that you can discharge batteries much faster than you can charge them. Also, it seems like you're imagining that you can charge batteries with more power if you charge them in parallel instead of series. This isn't true since you're just trading a higher voltage/lower current for a lower voltage/higher current.
    - DenisM 6y ago
      No free lunch for me then. Thanks for explaining.
coryrc 6y ago
Not a problem, just expensive. Wires running to each individual would have to be larger than ones used just for sensing or small charge bleeding. I was considering a design where there was a PCB on every cell with an individual charger, but it's just more cost and parts to break, plus no redundancy (unlike having two big chargers in parallel).
- DenisM 6y ago
  How about a middle ground then? Let's make a few 3-series batteries and charge them. When done charging we stack four of them together to get a single 12-series to power an e-bike. Would that allow for much faster charging?
  - Johnythree 6y ago
    No. Charging in Series or in Parallel still requires the same amount of power.
- tshaddox 6y ago
  I don't think it's very expensive. Nearly all hobbyist RC drone batteries that are have 2 or more cells in series have a big discharge lead with all the cells in series, plus a smaller parallel "balance port" that provides access to each individual cell. Balance ports are used for charging, and can also be used in flight to wirelessly report the voltage of each cell. There are plenty of common (and apparently somewhat reliable) balancing chargers for under $30. A popular $22 balancing charger for hobby RC drones: https://hobbyking.com/en_us/turnigy-accucel-6-50w-6a-balancer-charger-w-accessories.html https://hobbyking.com/en_us/turnigy-accucel-6-50w-6a-balance... A $15 sensor that plugs into the balance port and sends telemetry data about each cell: https://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-voltage-sensor.html https://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-volta... A similar sensor that beeps really loudly when any cell falls to 3.5 volts (these are very popular for people without telemetry systems): https://outofdarts.com/products/lipo-voltage-alarm-1-mini-size-buzzer-only-for-safe-operation-balance-port-connector https://outofdarts.com/products/lipo-voltage-alarm-1-mini-si... An article explaining balance ports in RC aircraft batteries: http://www.tjinguytech.com/charging-how-tos/balance-connectors http://www.tjinguytech.com/charging-how-tos/balance-connecto...
eecc 6y ago
I guess any fast charge pack has this, cars in particular. Pushing large currents thru strings of high impedance empty batteries must be dangerous and energy inefficient, while switching them to parallel configuration would make a world of difference. I'd be surprised if there weren't power routing circuits already available from major IC manufacturers.
- Johnythree 6y ago
  The only difference between charging them in series or parallel is that the Volts/Current ratio changes. The actual power required does not change..
  - eecc 6y ago
    Doesn’t it improve efficiency? Squeezing electrons through a string of exhausted batteries require current over a longer time and therefore more losses.
samatman 6y ago
Absolutely, and this is not unheard of in custom e-vehicle wiring. You have a discharge circuit, in series, and a charge circuit, wired separately to each cell. You're only using one at a time, as long as everything is properly grounded they don't get in each other's way. It's not really about charge time though: any vehicle battery is going to discharge faster than it charges, so it's practical to just dump a bunch of current across the pack if you're charging using the same circuit. What it does, is takes good care of each individual cell, and if you log some metrics you can detect underperforming cells and replace them. I will say that for most applications, charging off the series wiring, and adding an overvoltage module that shuts off charge to each cell when it's topped off, is going to be much simpler and good enough. But it can be done.

by DenisM 6y ago

PragmaticPulp 6y ago

A battery management system handles this. You still charge the pack in series, but the BMS will discharge cells that have a higher voltage than others. This results in all cells having similar charge levels. It doesn't change the charge time, though. You're putting the same amount of energy into the pack either way. 14.8V @ 100mA applied to 4 cells in series is the same energy as 3.7V @ 100mA applied to 4 cells individually. Can't escape the laws of physics. If power efficiency is critical (solar applications, for example), the BMS might have fancy switching circuitry to re-route energy from overcharged cells to undercharged cells. This is expensive, though, so most BMSs just use resistors to burn off extra charge from overcharged cells.

DenisM 6y ago
> You still charge the pack in series I understand that's how it's done, but can we charge faster if we DECIDE NOT to charge in series? I take it the answer is no? Imagine a series of 14 cells. I could charge them as a whole, but there is normally some limit on how much power I can pump into it. Now suppose that I disassembled the series and now I have 14 cells that I can charge independently before re-assembling them back. If I did that, would I gain anything? If nothing else this should prevent unbalanced charging, reducing overall heat of the pack and allowing higher current overall? I can see that some packs advertise 5a charge current and other packs of the same capacity offer 10a or even 15a. Wonder why is there such difference.
- reportingsjr 6y ago
  It would be marginally faster (best case probably a percent or two) for a much more expensive charging circuit. The reason for this is that you can discharge batteries much faster than you can charge them. Also, it seems like you're imagining that you can charge batteries with more power if you charge them in parallel instead of series. This isn't true since you're just trading a higher voltage/lower current for a lower voltage/higher current.
  - DenisM 6y ago
    No free lunch for me then. Thanks for explaining.

coryrc 6y ago

Not a problem, just expensive. Wires running to each individual would have to be larger than ones used just for sensing or small charge bleeding. I was considering a design where there was a PCB on every cell with an individual charger, but it's just more cost and parts to break, plus no redundancy (unlike having two big chargers in parallel).

DenisM 6y ago
How about a middle ground then? Let's make a few 3-series batteries and charge them. When done charging we stack four of them together to get a single 12-series to power an e-bike. Would that allow for much faster charging?
- Johnythree 6y ago
  No. Charging in Series or in Parallel still requires the same amount of power.
tshaddox 6y ago
I don't think it's very expensive. Nearly all hobbyist RC drone batteries that are have 2 or more cells in series have a big discharge lead with all the cells in series, plus a smaller parallel "balance port" that provides access to each individual cell. Balance ports are used for charging, and can also be used in flight to wirelessly report the voltage of each cell. There are plenty of common (and apparently somewhat reliable) balancing chargers for under $30. A popular $22 balancing charger for hobby RC drones: https://hobbyking.com/en_us/turnigy-accucel-6-50w-6a-balancer-charger-w-accessories.html https://hobbyking.com/en_us/turnigy-accucel-6-50w-6a-balance... A $15 sensor that plugs into the balance port and sends telemetry data about each cell: https://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-voltage-sensor.html https://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-volta... A similar sensor that beeps really loudly when any cell falls to 3.5 volts (these are very popular for people without telemetry systems): https://outofdarts.com/products/lipo-voltage-alarm-1-mini-size-buzzer-only-for-safe-operation-balance-port-connector https://outofdarts.com/products/lipo-voltage-alarm-1-mini-si... An article explaining balance ports in RC aircraft batteries: http://www.tjinguytech.com/charging-how-tos/balance-connectors http://www.tjinguytech.com/charging-how-tos/balance-connecto...

eecc 6y ago

I guess any fast charge pack has this, cars in particular. Pushing large currents thru strings of high impedance empty batteries must be dangerous and energy inefficient, while switching them to parallel configuration would make a world of difference. I'd be surprised if there weren't power routing circuits already available from major IC manufacturers.

Johnythree 6y ago
The only difference between charging them in series or parallel is that the Volts/Current ratio changes. The actual power required does not change..
- eecc 6y ago
  Doesn’t it improve efficiency? Squeezing electrons through a string of exhausted batteries require current over a longer time and therefore more losses.

samatman 6y ago

Absolutely, and this is not unheard of in custom e-vehicle wiring. You have a discharge circuit, in series, and a charge circuit, wired separately to each cell. You're only using one at a time, as long as everything is properly grounded they don't get in each other's way. It's not really about charge time though: any vehicle battery is going to discharge faster than it charges, so it's practical to just dump a bunch of current across the pack if you're charging using the same circuit. What it does, is takes good care of each individual cell, and if you log some metrics you can detect underperforming cells and replace them. I will say that for most applications, charging off the series wiring, and adding an overvoltage module that shuts off charge to each cell when it's topped off, is going to be much simpler and good enough. But it can be done.