Solar charger current divider/regulator?

[Fred Bloggs]

Hi Fred

How about a modification to your circuit along the following lines:


- +----+ +
+-----| PV |------+-----------------------+
| +----+ | |
| | |
| e e
| \| Rb Rb |/
| |-+--^^^^-+-^^^^--+-|
| /| | | | |\
| c / | / c
| | \Rf | Rf\ |
| | / | / |
| | \ | \ |
| |___|_______| | |
| | | ____________| |
| | | | |
| +-------------+ | | +---------+
| | |___|______ |
| | | | |
| | | | |
| | | | |
| | | | |
| | \ * | | * \ |
| +------|<|----+ | | +----|>|--+-----+
| | \ | | | | \ |
| | 13.0V | c c | 13.0V |
| | _ | |/ \| | _ |
| ----- _| +-----| |------+ _| -----
| --- | |\ /| | ---
| | / e e / |
| | 1k | | 1k |
| | / | | / |
| | \ | | \ |
| | | | | | |
+---+-------------+-------+------+--------+---------------+


* active zener's

Would need to give some thought to the required values for Rb/Rf to
provide orederly change-over, but I reckon this would do the job.
(Nice thing is it makes the PNPs work for their living and handle the
change over as well as defining the two charge currents).

Regards

Ted

That is a nice improvement. It turns out the current mirror fails when
the batteries are at substantially different potentials, causing one
transistor to saturate. So something needs to be done to set the common
base potential to the greatest collector potential when currents are in
balance, and set it to the least potential when all the current is to be
diverted to the uncharged battery. Something like this:

Please view in a fixed-width font such as Courier.



- +----+ +
+-----| PV |------------+------------------------+-----+
| +----+ | | |
| Re | Re | |
| +--/\/\------+--/\/\--+ | |
| | 6.8 6.8 | | /
| | | | 1k
| e e | /
| \| |/ e \
| T1 |----------+------| T2 \| |
| /| | |\ T3 |---+
| c --- c /| |
| | D7 \ / | c /
| | --- | | 1k
| | | | | /
| | | | | \
| +-|<|--+---|>|------+---|<|--+---|>|-+ | |
| | D1 D2 | D3 D4 | | |
| | | | D5 | |
| +-------------------|----------------|--|<|--+ |
| | | | | |
| | | +--|<|--+ |
| | | | D6 |
| | / | |
| | 1k | |
| | / | |
| | \ | |
| ----- | ----- |
| --- c --- c
| | |/ | |/
| | S1 --| 2N4401 | S2 --| 2N4401
| | |\ | |\
| | e | e
| | | | |
+---+-------------------+----------------+-------------+


T1,T2,T3 D45H11 TO-220

When both batteries are undercharged, the circuit comes up with S1 ON
and S2 OFF. D1-D4 and D7 ensure that T1 and T2 are conducting equal
currents regardless of the individual battery voltages. When the first
battery hits full charge voltage, circuit state changes to S1 OFF and S2
ON. This turns T1,T2 OFF, and T3 saturates ON. Diodes D5,D6 divert all
of T3 current into the undercharged battery. When the last battery
charges to limit, S2 is turned OFF and the circuit goes into idle state.
T1 and T2 should be tightly coupled thermally, on the same heatsink in
close proximity- maybe 10W capacity and/or thermal cutout. The diodes
can be 1N5822's, and D1-4,D7 should be thermally coupled also. The
D45H11's are fairly cheap at $0.90 ea at http://www.mouser.com P/N
511-D45H11. The threshold can be cheap too- have not decided how that
should play out yet.