Here are several unrelated items, all concerning small "smart" battery chargers.
(1) Several past threads here discussed how to hook up single NiMH cells of size AA or whatever to charge off a solar panel etc. My favorite approach is to focus the solar (or backup power) system efforts on a larger 12V battery or batteries, and use the 12V to charge small cells via an appropriate smart charger. My favorite charger for that purpose is the BC1HU which has been around for some years under different brand names and is still available for less than $30. It has two main relevant features: (a) it runs on 12 VDC (from the included wall-wart - or from any 12V battery), and (b) it can charge AAA, AA, C, D cells individually or in any combination - it handles each cell separately. This is important if you use odd numbers of cells to run some things, e.g., a single-AA radio or a 3-AA LED headlamp, since most chargers can only charge them in pairs. My only gripe with the BC1HU charger is that it makes the cells fairly warm, I wish I could tell it to use a lower current. It does adjust the current to the size of the cell, e.g., it gives AA cells more current than AAA cells, but all sizes get quite warm, even D size.
(2) Devices that won't run on the 1.2V NiMH cells: instead of throw-away alkaline (1.5V) cells, the rechargable alkalines, once sold under the "Renewal" brand, are still available under the "Juice" brand. One needs a special charger for them. The "Juice" place sells several models, one of which can also charge NiMH (and NiCad) cells (auto-detecting) and can charge up to 4 AA or AAA cells independently (each can be a different type or size). Alas its wall-wart is 3V not 12V, so not suitable for running off of a 12V non-grid system.
(3) Charging high-voltage (more than 12V) systems: there are smart chargers for them, but they're harder to find and also may not work reliably once the batteries get old and the cells are not perfectly balanced. Suggest dividing the pack into series sections, e.g. a 24V pack can be thought of (and often is built as) 2 12V packs in series. Can wire a connector for each section for separate charging with a lower-voltage charger. A bit more hassle but doable.
(4) Smart chargers for small sealed lead-acid (SLA) batteries: these are a bit harder to find these days than those for NiMH? I bought one that is small and light and outputs 12V 0.8A. Recently hooked it up to a 12V 12AH SLA that was mostly full, and it ran on its higher-current mode for a while (an hour perhaps?) before switching to the float mode. (Once can tell by the color of the LED on it.) During that time I happened to hear that the battery was making some sounds like it was venting. Checked the voltage and it was about 14.8V. Seems a bit on the high side? It did drop to about 13.6V in the float mode. Is this battery too large for this charger (causing too long a delay before it decides to switch to float mode)? Is there a smart SLA charger on the market that can be used on a variety of battery sizes, say from 3AH to 12AH (all 12V)?
(5) Recently I got another very small (0.4A) smart SLA charger that came with a lawn mower that has electric start (12V 3AH battery). Seemed like a nifty little thing, LED green with just AC power connected or in float mode, red when charging. Alas the plastic case of the cheap thing came apart the second time I unplugged it from the wall, and in the process broke the 3-wire 2-color LED off. Thought it would still work without the LED indication, but no, it now shows zero output current. Why would that be? Tried attaching other LEDs to the wires, I get the power-on glow but still no output current. The output wires show about 1.5V DC to a voltmeter (no battery connected).
(6) Charging SLAs without a smart charger: I have some "constant-current" (unregulated) chargers and use them for an initial bulk charge, stopping it manually. Then I hook up a constant-voltage float charger (homemade with a simple 3-pin 12V voltage-regulator IC with 3 diodes added in its ground wire to raise the voltage to about 14.2V) for some hours / overnight (not longer). Not optimal but workable. Not sure what the optimum voltage is and how long to charge. I used to have 2 diodes in that circuit, making it about 13.5V, as that's a standard "float" voltage, but my usage is not really "float", rather, a slow top-off.
