Solar cell charging 4V Pb-acid battery

[Phaedrus]

I'm working on a project run from a 4V 4.5Ah lead acid battery (EnerSys Cyclon series) that needs to recharge via solar panel. The solar panel will be a fixed voltage, but I haven't chosen it yet; I can choose anything between 4.5V and 18V. I'm hoping to find a module for this application, but most lead-acid solar panel charger modules are designed for 12-24V, not 4V. If there's no efficient module, what's the best circuit topology to use?

The battery, by the way, will be driving a couple of low-power analog circuits and a 1.8V buck regulator powering an MCU. ~5mA idle load, ~25mA pulse load from the MCU and related circuitry every .25s, occasionally a ~50-60mA pulse.

[tautech]

I have been doing similar using NiMh.
It all depends on the power of the PV panel.
In my case 6 V (nominal) 100 mA was easy with only 0.6 W to regulate. Series diode and zener diode. Extra series diode/s if necessary for further 
Series diode reduces leakage and trims zener voltage, bit of trial and error to get zener fully on at the desired max charge voltage.

[HackedFridgeMagnet]

You might be able to modify somebody else's buck style charger, through the resistors in the feedback network.

Things to watch out for with solar powered long term projects.
Low battery cut out. I've seen devices destroy their batteries in the field every winter because they didn't have this.
Temperature compensated charging. If it's over 40 C then pls stop. Also seen batteries destroyed because of the lack of this.
Reverse polarity protection.
Fuse the battery.

[Seekonk]

For a light load I would use a LM2596 regulator.  These completed modules are available on ebay shipped  from China for only about $2.  In most cases these will work fine right out of the box with 12V panels.  They can be operated at power point of about 18V for maximum power with the following circuit which uses the on off pin #5.  Any variety of voltage sensors could be used.

[Phaedrus]

You might be able to modify somebody else's buck style charger, through the resistors in the feedback network.

Things to watch out for with solar powered long term projects.
Low battery cut out. I've seen devices destroy their batteries in the field every winter because they didn't have this.
Temperature compensated charging. If it's over 40 C then pls stop. Also seen batteries destroyed because of the lack of this.
Reverse polarity protection.
Fuse the battery.

Re: Temperature Compensation --

The device needs to function over a wide temperature range: -40C to +65C. I don't need it to charge over the full range; however, it will probably need to charge down to -10C and as high as 45-50C. Enersys does rate the batteries for this temperature range, and suggests a +/-3mV NTC compensation, but warns of reduced life expectancy. I will probably cut charging outside of the temperature range above though.

[David Hess]

Just keep it simple then.  Implement a buck converter with temperature compensation as part of the output voltage feedback loop and a separate output disconnect for the load if the battery voltage drops.

Since you only have two cells, I would consider charge balancing as well in lieu of a controlled overcharge because it is easier to do.

[sacherjj]

If you are needing to maximize your power transfer, solar power tracking charger chips would be the best bet.  These pull only as much current as keeps the maximum power transfer from the solar cell, before buck or boosting into the battery. 

If you are just trickle charging or keeping topped, then buck circuit seems to make the most sense.