i have encountered this scenario many times. imo effects vary depending on usage. what you have to remember is your battery is not going to get fully charged through one charge cycle with an under rated charger despite many manufacturers claiming all that would happen is the charge cycle would be extended. i had one site i took over with under rated ah chargers, the trucks would overheat occasionally and have to rest, cooling fans were on the trucks along with holes cut into covers in an attempt to cool the trucks off better. after we replaced the chargers with ones that were matched to the batteries ah rating the fans were removed, battery shift life was increased and truck breakdowns decreased, they were running much cooler. i have also seen under rated chargers at sites with one truck they use an hour a week with a 10 year old battery working fine for the hour they need it for. will a smaller ah charger damage the battery? no, but it will decrease your batteries capacity over time. if this is your truck a battery hydrometer may be a good investment.