It's always best to match batteries to the same amphr rating. IE a 240 amphr Trojan 6V golf cart battery circuit should have a similar +/- 20 amphr start battery. I prefer to also keep all the batteries the same brand or at least have the same factory settings for charge/float etc. We just changed out our Trojans for 251 amphr 6V Interstate and a 12V Interstate marine start/house battery. This allows the batteries to absorb equally putting less strain on the charging system.