A lead-acid battery on charge can make heat in three different ways, and only some of them are faults. A battery that is slightly warm at the end of a normal charge is not alarming; a battery that is hotter than last month, or one cell that is hotter than its neighbours, is telling you something. This post separates the three sources so that a warm battery becomes a measurement and not a worry. We have drawn on Crompton’s Battery Reference Book and, for the practice, on the traction-charging passages of two reference works; the limits for your cells are your battery maker’s to give, and we state none.
Source one: the reaction, and the resistance
Crompton shows that two opposite effects act in a cell during charge. The charging reaction itself absorbs heat, which tends to cool the electrolyte, while the charging current passing through the cell’s internal resistance produces ohmic heating, which warms it. Whether the cell warms or cools depends on the balance. In his worked example for a two-volt cell, cooling occurs only at relatively low charging currents, of the order of 20 to 50 amperes, and he finds cell heating would not be a serious problem at currents under 200 to 300 amperes for cells in a resistance range of 0.002 to 0.027 ohm, once natural heat losses are allowed for. The practical message is reassuring: the main charge of a healthy battery on a sensible charger should not heat it much. Heat rises with the square of the current, so a charger driving a high current into a high-resistance, aged battery is where this source starts to matter.
Source two: charge that has nowhere to go
Energy put into a battery that cannot be stored must go somewhere. Near the end of charge, and in any overcharge, the current no longer converts active material, and most of it goes into splitting water into gas and into heat; that is our reasoning from the basic chemistry, not a figure from the book. It explains Crompton’s caution about traction batteries: too many opportunity-charging periods in a shift may cause unacceptably high battery temperatures, and a battery worked for eight hours, charged for eight and worked again straight away will have its life considerably reduced because it never gets a chance to cool. Our guide to opportunity charging covers that practice, and the charger side is in our guide to choosing a forklift battery charger.
Source three: stray paths on the cell tops
Crompton’s third source is local. If acid or electrolyte spilt on the top of a battery links cells that should be isolated, current finds a low-resistance path between them, and that path heats where it is thinnest. His example is the electric locomotive battery in a coal mine, where acid and coal dust make a slurry that conducts well enough to cause intense local heating and, in some circumstances, ignition. The same mechanism applies to any wet, dirty battery top in any industry, with less drama. Overfilling, a cracked vent plug or a spillage during watering all create it; our guide to battery water covers the watering round and our guide to DC earth faults explains how a leakage path shows up in a measurement.
Why the heat matters
Heat shortens the life of a lead-acid battery, which is why this is worth measuring; our guide to operating temperature explains the trade between capacity, which a warm battery can deliver, and life, which it loses. The charger’s settings matter too, and some chargers adjust their output for temperature; ask your maker whether yours does, and what limit applies to your cells.
Sealed valve-regulated batteries deserve extra care. They have less electrolyte to carry heat away and recombine gas inside the cell, so a battery that runs hotter on charge can draw more current, which makes it hotter again; our guide to thermal runaway in VRLA batteries explains the loop. For a flooded traction battery, ordinary watering and a rest after charge are usually enough, but the same habit of logging end-of-charge temperature applies to both.
What to do with a warm battery
Do not guess by hand. At the end of a normal charge, take the electrolyte temperature of a few cells with a thermometer or an infrared reading of the cell wall, write it in the battery log with the date and the ambient temperature, and look at the trend, not the single reading. A reading that rises month on month points to overcharge, a charger setting, a hot room or an ageing battery. One cell that is consistently warmer than its neighbours deserves investigation by a person who can test it, since it may be failing; a wet top deserves cleaning to the maker’s procedure. And if the battery is worked immediately after charge, give it the rest that Crompton recommends.
Tell us the battery, the charger and the readings you are seeing and we will say plainly whether they look normal or not.
Sources quoted: T. R. Crompton, Battery Reference Book, 3rd ed. (Newnes, 2000), chapter 1, section on heating effects in batteries, and chapter 48, sections on opportunity charging.