Current limiting module for add-on battery pack - ideas?

[alcatraz]

Hey

I've been thinking about adding some more capacity to my gotway/begode.

I'm thinking about using the charge port and making my own 84v pack. I need a way to set a hard current limit of 5A or whatever so I don't burn the charge port. Got any ideas?

I don't need a dc-dc board really because the add-on pack is the same voltage. I'm trying to keep efficiency high. Don't want heat to build up in a converter or anything.

But I also don't want to match voltages with the packs in the wheel. Therefore I'm thinking current limiter and using the charge port. Simplest and most straight forward.

[Planemo]

I honestly think the easiest way to do this is to simply run an extra internal pack, as I did with my 84v MSX. Trying to run an extra pack at a different current will be a headache I feel. I know I can get a range extender battery for my ebike which I think works the same way you suggest, but theres a fair bit of trickery involved afaik.

[Chriull]

1 hour ago, alcatraz said:

Hey

I've been thinking about adding some more capacity to my gotway/begode.

I'm thinking about using the charge port and making my own 84v pack. I need a way to set a hard current limit of 5A or whatever so I don't burn the charge port. Got any ideas?

A dc dc converter with 5A current limit, as usable for li ion charging.

1 hour ago, alcatraz said:

I don't need a dc-dc board really because the add-on pack is the same voltage.

Maybe at the start - as you limit the current for the external battery it looses less capacity, hence voltage for some time. Just inbetween it recharges the internal battery and "equalizes" the voltages again.

1 hour ago, alcatraz said:

I'm trying to keep efficiency high. Don't want heat to build up in a converter or anything.

But I also don't want to match voltages with the packs in the wheel. Therefore I'm thinking current limiter and using the charge port. Simplest and most straight forward.

The basic law here is Delta U = R * I. R is given by internal resistances, wires, contacts, etc. Current shall be up to 5A, so the variable value for control is delta U - the voltage difference the DC DC controller transforms the external battery voltage above the internal battery voltage...

[RagingGrandpa]

4 hours ago, alcatraz said:

[add capacity to an 84V EUC] using the charge port and another 84v pack

I did that...

The boost converter is nice because it ensures the backpack gets depleted all the way below 3V/cell, while the EUC pack remains ~3.6V/cell. So you keep the EUC in its higher-voltage, higher-speed-capable state for longer, than without the converter.

But as you can see from the schematic, the current is not limited if the backpack voltage is higher than the EUC, because it can pass right through the converter diode. So I can only connect the backpack if the EUC is fully charged (e.g. within 1V of the backpack). 
To limit current during mismatched voltage, you'd need a buck mode- a more complex converter.

Fuses melt and prevent user error from damaging components.

(Ignore the GX16 part of the diagram... I put one on the backpack to allow recharging it from a standard Gotway charger.)

On 9/21/2020 at 9:18 AM, RagingGrandpa said:

https://forum.electricunicycle.org/topic/20039-raginggrandpas-charge-and-ride-greenworks-backpack/

[alcatraz]

Thank you all.

I've been looking at some dc-dc modules here in China. 

Only two seem to have some potential. I'm not sure how they're going to like regenerative braking.

This one has a 10-110V input and 0-100V output up to 10A/1000W. ~20usd

 

Or this one for about 10usd. 

 

Also a third option is the lazy option, but also the most efficient (in a way).

It's to just pop out my 850Wh Tesla v1 pack and hook it up to an XT60 poking out (of the MSX) and taped over. Then I don't need to carry a dc-dc converter, or buy new cells, or worry about efficiency or something heating up inside my bag.

[Planemo]

1 minute ago, alcatraz said:

Also a third option is the lazy option, but also the most efficient (in a way).

It's to just pop out my 850Wh Tesla v1 pack and hook it up to an XT60 poking out (of the MSX) and taped over. Then I don't need to carry a dc-dc converter, or buy new cells, or worry about efficiency or something heating up inside my bag.

This would be my solution.

Or, given you suggested you could make packs, knock up a 259Wh pack (same as was used in my 1860wh 84v MSX) in a 20S1P config and just slip it into your backpack. Far lighter than an 850Wh pack and still gives a decent increase in range. Or, given the size, fit it inside the wheel itself

 

[RagingGrandpa]

7 hours ago, alcatraz said:

I've been looking at some dc-dc modules here in China. 
Only two seem to have some potential.

Both of those have digital controls, which is a pain because you have to adjust them carefully during use, and if any of the buttons get pressed, it will mess up the output.

2nd one also says input range to 60V, so you can't use it with your 84V pack. 

7 hours ago, alcatraz said:

pop out my 850Wh Tesla v1 pack and hook it up to an XT60 poking out (of the MSX)

Try connecting it to the charging port instead (after matching the battery voltage). 
The charging port is safer, because it has overcurrent protection (around 20A for MSX). It means you can short the connector or cables while riding, and it won't cause the EUC to turn off.
I use my MSX backpack that way. The position of the charging port on MSX is great: the connector stays inserted while riding, and pulls out easily if you snag it or fall. (Remove the threaded ring from your GX16 cable, to ensure it can pull out of the EUC reliably.)

Check that the port is double-pinned, so it can sustain 10A without overheating. Early MSX were single pinned; during 2019 production they started using all 4 pins.

Be sure to add a ~15A fuse in your auxiliary pack, to be sure that the same cable short doesn't start a fire in your backpack :/ 

[alcatraz]

16 hours ago, RagingGrandpa said:

Be sure to add a ~15A fuse in your auxiliary pack, to be sure that the same cable short doesn't start a fire in your backpack :/ 

Did you ever blow this fuse out on a ride?

On the double pinned charge ports, can I simply measure 0 ohms on two pairs of pins and determine it is? Hmm...

[RagingGrandpa]

7 hours ago, alcatraz said:

Did you ever blow this fuse out on a ride?

Yes.
I have a boost converter, and had it adjusted a little too strong... brisk acceleration at the start of the ride drew too much current and opened the fuse. 

Without the boost converter, I think you'll have less current draw, so 15A is a good place to start.
 

7 hours ago, alcatraz said:

double pinned charge ports, can I simply measure

Easier than that- measure voltage between pins 3 and 4.
Double-pinned connectors will show 84V, just like across pins 1 and 2. 
(Single-pinned connectors will be open circuit at pins 3 and 4.)

Probe carefully with insulated tips!
(But if you slip and short the pins, MSX BMS should save you from damage...)