Charging 2x 4S packs as 8S on BC8

[aaron07]

I am charging 2x 4S packs as 8S on my BC8. After flying about 74% of my pack (1554mha) my charger reads my pack as 7 cells instead of 8. The individual cells are about 3.70, but if they are 3.80 the charger reads the two packs in series as 8 cells.

Can someone please explain why my charger reads the packs as 7S.

I am tired of charging some packs individually. I have 9 8s packs / 18 4S packs....so much charging

[aaron07]

This was a PM from RC Accessory for those who are interested.

"5 Amps would be the max that the BC-6 will charge a 8S pack at. Using 2200 packs, that would be 2.2C.

So for the PS to feed min of 150 watts, you multiply the voltage out of the PS (13.8V) x the amp rating of the power supply. The higher the voltage of the PS the better. The BC-8 can take up to 18V input.

So a 20 amp PS would be 13.8V x 20 = 276 watts before any efficiency loss. You need no less than 300 watts. I would recommend a 30 Amp PS. 13.8V x 30 = 414 watts.

In reality, you would be maxing out a 30 Amp PS, unless you get a higher voltage unit. If you got a 15V 30 Amp model, that would be 450 watts."

Thanks RC.

[aaron07]

This was kgfly, awsome info aswell:

You have at least two choices. Either a single high-power supply capable of running 2xBC8 at the same time or two mid-range supplies, one for each charger.

Charging 4s 2100 at 2C requires about 65W output
Charging 8s 2100 at 2C requires about 130W output

The BC8 has a 150W/7A output rating so for full power operation you should budget at least 200W input power per charger but a bit more headroom will ensure that the power supply is not maxed out and will last longer.

For 200W you would need at least: 12V/17A, 13.8V/15A, 15V/13A
For 400W you would need at least: 12V/33A, 13.8V/29A, 15V/27A

So Evan's recommendation to look for 15V/30A = 450W power supply is very good advice.

400+W power supplies are harder to find, although they are used more commonly in the ham radio domain and can be found from those sorts of suppliers.

Here are some examples I came up with googling "15V 30A DC site:au":

Unboxed SMPS
This is the cheapest approach but you have to wire them up yourself. If you want a case, on/off switch/light then you have to build it yourself.

15V/500W ~$180 http://www.computronics.com.au/meanwell/sp-500
15V/480W ~$200 http://www.computronics.com.au/meanwell/sp-480
15V/32A ~$330 http://users.tpg.com.au/users/p8king/s100.htm

The prices from computronics look pretty good to me.

DC bench supply
This looks better, 15V/30A (450W) properly packaged bench supply ($375): http://www.wiltronics.com.au/catalogue/shop.php?cid=520

Or you can simply get two 13.8V/20A supplies for $99 each from Jaycar or DSE.

Having a single 15V/400+W supply provides a degree of future-proofing in case you change to a single 250-380W charger in the future (eg Hyperion 0610iDUO or Cellpro10s) but the pair of 13.8V/20A units will certainly do the job for your current 2 x BC8 setup.

Batteries
For field charging with high-power chargers you need a deep-cycle battery designed for sustained loads and being regularly discharged by about 50-60%. A conventional car battery (known as a cranking battery) is not well suited to the job and will die fairly quickly if regularly discharged by 30%. You would probably need at least a 120AH AGM deep-cycle battery to run two high power chargers at the same time.