The battery charger is usually sized according to the type and size of the battery bank it is charging. Having too small a battery charger is as bad as having too large a one. Buying a "cheap" charger is generally a really bad idea, since it will often shorten the life of the batteries.
What you need is a good three-stage smart charger. This means that the charger will go through the bulk, absorption and float phases of charging and step the voltage and amperage up/down as the charge level on the battery changes.
You don't say how many amp-hours your battery bank is or what chemistry they are. As a general rule, the battery charger should be 20-25% in amperage capacity of the 20-hour amp-hour rating of the battery bank. For instance, if you have a house bank of two Trojan T105 batteries, that would have a capacity of 225 amp-hours, you'd want a 40-55 amp charger or so.
However, AGM batteries usually have a much higher current acceptance rate, especially some of the newer, more modern technology batteries, like the Optima Spiral cell batteries. In the case of these batteries, you might want a larger charger, due to their higher current acceptance rate in the bulk phase of charging.
You also want a battery charger that can be set for the voltage requirements of the battery chemistry that you're using—wet cell, AGM or gel. Not doing so can drastically shorten the life of the batteries as well.
A good brand of "smart" battery chargers are the Iota brand. They're quite reasonably priced and given their price, it doesn't really make much sense to get a "cheap" 10-amp charger. The 30 amp charger is about $130 and their 40 amp unit is only $145. See this website
for more information regarding them.