DEXT16 maximum number of DSSR's

[David Burton]

I understand the documented limit for the DEXT16 is 16 DSSR's, 8 per RCBatt+, but with my configuration it would really be helpful if I could push it to 18, or 9 per RCBatt+.  I have a 54 x 250W 60-cell panel configuration that I was hoping to have configured with dual PV arrays, 18 and 36.  18 isn't an issue, but 36 of course requires 18 x DSSR's.  All 27 DSSR's are very close (<3ft) from the DEXT16.

[Dacian Todea]

David,

Will you be using diversion or just dual PV array for charging ? 

The RCBatt+ is based on a 10mA constant current limiting circuit (two outputs each with his own 10mA current limit).

The DSSR20 has an equivalent 7kOhm input resistance on the Batt remote connection

So with 10mA / 8 = 1.25mA for each DSSR20 thus 7000Ohm x 0.00125A = 8.75V well above 5V so not a problem and having 9 should also not be a problem.

For RCHeat is a bit more problematic if you will use that for all DSSR20's as each DSSR20 will use around 10mA on Heat input so it will add up to 180mA instead of 160mA max design but again the DC/DC converter may run a bit hotter but should work.

[David Burton]

Yes I do plan on using diversion, but likely not all of them.  Thank you so much for the details Dacian, this will keep my design more simple.

[Casey]

Not that I expect to ever want more DSSRs, but I am curious why the SBMS0 has maximum limit of 30 DSSR20s?

[Dacian Todea]

Casey,

Each DSSR20 requires around 2mA as control and so each EXT IOx can handle about 15 DSSR20

Also 30x DSSR20 will result in about 600 to 660A of charge current continues depending on panels and can peak around 750A the max of what SBMS0 can measure anyway.

With dual PV array setup where not all panels are connected at the same time up to 48x DSSR20 can be used 32 + 16

The 750A limit is related to the way software is designed and the compression method used with base 91

Humans work in base 10 so if you set a limit of 3 digit for your value then you can display max 999  (10x10x10=1000)

In Hex base 16 with say two characters "FF" you can display max 255 (16x16=256)

In base 91 that I use and my limit of 3 characters I can store a value no larger than (91x91x91 = 753571) thus I limited the max current to 750.000A with mA resolution.

There are methods to increase this keeping the 3 characters limits but for now I think 750A is sufficient.