Opposite way around Jon, the load terminal is pulled positive by the resistor between battery positive and the load terminal and this pulls the yellow wire and terminal 3 on the SSR positive. The T1 Lithium BMS controls the SSR terminal 4 negative and can open that circuit so terminal 4 is floating.jon_d wrote: ↑Sat May 09, 2020 9:40 amThanks. The load terminal controls the SSR and the SSR is pulled off via the resistor when the load terminal is "off". (floating using an open collector circuit.)T1 Terry wrote: ↑Fri May 08, 2020 5:17 pmThe Plasmatronics controllers can be programmed for multiple different functions. One function duplicates the control the unit uses internally to control the solar to limit the battery voltage. By setting the load terminal on the PL20 to be open circuit when not trying to limit the solar charging current and PWM linking to the battery negative when the PL20 starts to try and limit the current from the solar, this terminal is either floating or PWM switched negative. By placing a resistor between battery positive and this terminal, when the terminal is floating (not trying to limit the solar charge current) the resistor forces this terminal to be battery positive. By connecting this terminal to the control positive of a solid state relay the PL20 is supplying a positive signal/supply to remote control the solid state relay. If the PL20 wants to limit the current from the solar to control the battery voltage, it pulse width modulates the time the load terminal is pulled negative. The resistor just becomes a miniature heater element when this occurs because battery positive is supplied to one side and a varying negative voltage on the load terminal side and in turn the terminal is pulled to a lower and lower voltage until it is not enough to fully open the SSR and eventually fully open the SSR that cuts the flow of current across the load terminals of the SSR. As many SSR's as required can be driven this way, if the total load exceeds 20 amps, the max load for the PL20 load terminal, an intermediate SSR can be used so the PL20 control that relay and in turn it controls all the other solid state relays.
The PL20 is the control but the capacity of the bank of SSR's is the current limit, I've run 3.8kW of solar into a 12v 700Ah lithium battery using this set up and 5.2kw of solar into a 24v 600Ah battery on another off grid install. The only problems I've ever had was I once used a cheapie SSR bought from evil bay that was supposed to handle 220vdc at 100 amps. It actually caught on fire while controlling 30 amps & an open circuit voltage of 120vdc and caused serious damage to the rest of the control board. A rather expensive and embarrassing lesson not to ever do that again. The SSR's from RS components we use for that purpose these days cost over $150 each and we have to buy them in bulk to get that price, so we have developed a different control method when these voltages are involved.
The other side of the control on the SSRs is controlled by our T1 Lithium BMS as a secondary charge control if a cell goes high voltage while the battery voltage is still below the PL20 control threshold.
These solid state relays can produce a bit of heat when in current control mode, so we mount them on heatsink. If there are multiple SSRs involved, we build a suitable tunnel heatsink, add 50*C thermostats above each SSR and use them to control a m
Maglev fan to pull air through the tunnel heatsink to avoid overheating the SSRs. Very careful selection of the SSR used is imperative, minimal voltage drop across the FET is a must or the heat can not be controlled simply using air, it would need a water cooled heatsink and that is just over the top for such a charging system. The losses are serious as well if the wrong SSR is used.
T1 Terry
With the BMS supplying a battery negative to terminal 4 and the battery positive via the resistor to the load terminal to terminal 3 via the yellow wire, the SSR now sees a voltage across the control and pulls the SSR closed allowing the solar to flow SSR load terminals.
If the PL20 wants to stop the charging it closes the negative circuit inside the PL20 and pulls the load terminal negative. The resistor now becomes a heater element, the yellow wire to terminal 3 is pulled negative resulting in no voltage appearing across terminal 3 and terminal 4, the SSR opens the load terminals and current flow from the solar stops.
If the BMS senses a cell going into run away it opens the circuit to terminal 4 so now there is a positive supply from the PL20 to terminal 3, but no negative supply to terminal 4. Again, there is no voltage seen across terminal 3 & 4 so the SSR opens the load circuit resulting in no current flow between the solar and the battery.
T1 Terry
