Thermal Runaway in batteries
When they don't hold a charge very well, it means the risk of one cell becoming shorted increases which lowers the string voltage and turns the cel from a capacitor into a resistor and this can make one cell runway with self heating with sufficient current from I^2R for R_shunt losses rather than ESR series losses.

Of course you can reduce the charging current with a large series R, ask if the units have temp compensated Charging voltage and se if there is a "de-sulphation pulser add-on device" that draws current every 20 to 50us and dumps it back into the battery in &lt;50ns to breakdown lead sulphate crystals.  That works in some cases but dendrite lead growth from undercharged or overcharged battery cells can cause whisker shorts.  

Good batteries come in clear plastic poly-carbonate cases so you can see what's going on.