>I have a single-item application that requires battery backup. I am > planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them > in > trickle charge permanently. Simple limiter resistor will put the trickle > charge at 20mA max (<1/30C) which is far below the 1/10C most people > comment > as acceptable for trickle charge. The NiMh are high quality Kan batteries. I have some experience in this - but the information from reputable manufacturers provides good guidelines. You'll find that competent makers of larger capacity NimH strenuously note that NimH should NOT be trickle charged. Several degradation mechanisms are suggested but it seems that heating becomes more important with increasing density of reactants. It's interesting to note that some lesser know NimH makers (typically those from a very large country) still provide data sheets which seem just to be scaled up versions of older ones from other people. Given that this is not what the leading manufacturers do, you need to be careful which information sources you believe. However, older NimH and low capacity NimH do not have the same restrictions and modern very low capacity NimH even have suggested allowable trickle charge rates equal to the fast charge rates. As a rule of thumb it seems that anything under about 1200 mAh is OK for trickle charging and at 650 mAh I'd expect that you SHOULD be OK. Using a quality name brand is, of course, essential. Previously rates of C/10 have been recommended but in more recent data sheets and at higher capacity C/30 seems more favoured. My reading suggests that energy density per volume seems to be an indicator for typical cell shapes. If absolute energy density is not required then clamping the battery voltage at a level somewhat below full charge is liable to produce a safer result than ongoing trickle charging. This can be achieved with as little as eg a TL431 and 2 resistors. I have taken that approach in a design which needs to be as robust as possible while costing approximately $0.00 to implement. Thsis also extends battery cycle life, although that's not liable to be an issue in most backup applications. If you could accept the voltage range then a single cell LiIon battery may be a better choice. A simple LiIon charger can be very simple indeed while still treating the battery well - far easier to do than a simple cheap nimH charger. Indefinite LiIon float operation at a designed voltage somewhat below absolute max will allow indefinite standby lifetime operation with essentially zero current. Notional Vmax is 4.3V typically (depending on chemistry detail). Operation as 4.2 V is saferr at the cost of some capacity and 4.1 V or even 4.0 V allows very safe floating without having to worry too much about temperature changing the limits. Capacity at 4.0V would be well down on 100% but for a one off the energy density of LiIon would still allow a smaller and lighter battery than using NimH. Summary: Trickle charging modern low capacity NimH (Say <= 1200 mAh) at <