Best solution to charge 6V sealed lead acid from 9V MPP / 10.8V O.C solar panel

Hi Oli

Not sure I'm really understanding your problem but do have some experience with low power 12V installations. My understanding is that the solar cell (within the bounds of it's maximum O/C voltage) behaves as a constant current generator whose output is proportional to the light intensity.  Therefore, connection to a lead acid type battery may simply achieved using for example a LDO low quiescent current linear regulator to limit overcharging, crude but simple. For example I've used an LP2952 with output blocking diode (shottky). Losses are the volt drop in any non return diode(s), the LDO forward voltage drop, the LDO quiescent current.

A far bigger problem fo me has been over discharging of the battery. This prompted a more elegant solution using back to back P channel MOSFETs controlled by a window comparator, disconnecting the battery from the charging and discharging circuit, preventing both overcharge and discharge. The window comparator and voltage reference/sensing are powered from the solar panel side of the MOSFETs, use low power devices, and achieve less than 1mA waisted in the circuit. Losses are largely confined to the comparator circuit quiescent current and reverse blocking diode in the panel and a very small MOSFET bias when the battery is connected to load/panel.

Hope this helps, good luck

James

Hi Oli, did say the LDO was a crude solution, ...no temp' compensation, which to be honest hasn't troubled automotive applications of Lead Acid batteries over the years, sealed Lead Acid are less forgiving. Quick charge/float charge not an issue for me as large capacity battery, so the 5W panel never generates enough current to be classed as quick.

Bear in mind the LDO will still pass current even though it's not reached the float/set voltage.

Might take a little while on the other circuit. The trick with under voltage is to bias the back to back P MOSFETs via a high value pull-up resistor on the battery side. The window comparator is powered from the other (load/charge) side. For the battery to be connected to load/charge, the dual P MOSFET switch gate is grounded via open collector, small current from battery through bias resistor. The window comparator is sensing the battery voltage on the load/charge side, when out of limits the open collector ground becomes open, the two MOSFETs gates go high to battery volts, then the window comparator circuit is isolated and no power is being drained from the battery (other than negligable leakage through the MOSFETs). The whole arrangement is then effectively latched off until the charging/load supply rises above the lower threshold. Note this may also be attempting to supply the load, which if greater than the panels current capacity will never occur, in which case you'll need an additional control circuitry to disconnect the load. In my case not a problem as the load is only a few mA.

A similar issue arises at overcharge, where the charge/load supply can suddenly rise to the panels open circuit voltage as the battery is disconnected, something to be aware of if your load is less than the panels current capacity, may require a dump load/clamping circuit to protect equipment connected. Not an issue if the load is greater than the panels output, as all current will be going to the load anyway and some from the battery.

Temperature compensation and variable charge rates are not considered as before, although could be. I'm generally using SLA batteries of 7.2Ahr to 19Ahr with 5-6W panels and very light but continuous loads, 4mA typically and tolerant of 18V input voltage. Installations have run without maintenance for several years, the last failure was due to water damage of the panel.

In case of bq2031 bq2031 you should take into account +5V regulator and op-amp supply currents. I have not found any information about efficiency but I think you can expect 80+ % as output voltage is low and diodes are used in buck converer. If there would be synchronous converter, you can expect better efficiency.

bq24450 is nearly same as UC2906. In standard application power switch driving current (DRVE pin) flows to load. Instead of reverse protection diode you can use p-MOSFET (lower voltage drop).