I'm building a simple constant-current charger (intended to charge at C/10 or less) for consumer NiMH cells (AA batteries). This is intended to help extend the life of some of my older cells that no longer charge in my smart charger. (Whether this is just due to age or "memory" I'm not sure, but if I discharge them to a fairly low value and then trickle-charge them for a day they seem to work well again in low-current devices.)
For the purposes of this question, assume that I'm starting with fully discharged cells (or at least equally balanced cells) and will be removing the cells from the charger after an appropriate amount of time (120%-150% charge).
(If you have comments on continuous "trickle" charging, this question is probably a better place to talk about that. Note that an earlier version of this question mentioned trickle charging in the sense of non-terminated charging; that seemed to be distracting from my main question here; thus the separate question above.)
My circuit is basically the one below (excluding the red part), which I've prototyped on a breadboard and it seems to work ok. But I'm wondering if it makes sense to add a second battery holder in parallel with the first (the red part below) so I could charge two cells at a time, albeit at a slower rate.
Though not shown in the schematic, I'm also considering adding a pair of diodes (probably 1N5819 Schottky, as I've got some of those on order) in front of the cells to prevent back-feeding, if that's necessary and won't cause other problems.
The idea is that this will divide the current between the cells, presumably charging at about C/20 if the cells charge at an equal rate, and in any case never delivering more than about C/10 to a cell since the total current is limited to that.
Will this work? Are the diodes necessary? Is it safe to charge cells in parallel like this? Will it avoid any sort of weird positive-feedback loop where a cell with greater charge actually starts drawing more current as its charge difference increases as compared to the other cell?