Guest RBall Posted March 16, 2005 Share Posted March 16, 2005 On 28/02/05 there was a posting by Pete C in which he stated " If you do fit two batteries make sure they are both (a) of the same capacity" Not being an electrical wizard could Pete C, or anyone, outline what electrical problems will be encounted if two batteries of different capacities are fitted. Link to comment Share on other sites More sharing options...
Guest Clive Posted March 16, 2005 Share Posted March 16, 2005 If you want to fit a second battery then fit one of the same size type and manufacture as the existing. Ideally of the same age as well. This will help ensure that both batteries share the charging and discharging loads equally. One reason for this is that when batteries age the odd cell can become short circuit due to lead based gunge in the bottom of the cell. With parallel connected batteries the "bad" battery will also discharge the "good" battery. If however you are going to fit two new batteries then it is best if you fit two identical 6 volt batteries connected in series. Sharing the load is ensured and none of the problems as detailed above. Link to comment Share on other sites More sharing options...
Guest RBall Posted March 17, 2005 Share Posted March 17, 2005 Thank you Clive for your prompt reply to my query concerning mixing leisure batteries of different capacities. I was concerned about the charging and discharging of two (or more!) "unmatched" batteries wired in parallel. Would I be correct in thinking that once the smaller capacity battery was fully charged that charging would cease and that the larger capacity battery would stay undercharged? Or would the charger continue to charge the higher capacity battery only? Similarly on discharge would there be a failure of supply once the smaller capacity battery was fully discharged? I, perhaps naively, thought that with the batteries being in parallel that charge would be transferred between them and that supply would continue until both were discharged. Even with a "matching" pair if one battery became faulty would not the other be affected? Link to comment Share on other sites More sharing options...
Guest Clive Posted March 18, 2005 Share Posted March 18, 2005 The vehicle alternator will just chuck out amps (within its limits) until it achieves 14 volts. How the batteries accept the charge is down to them. The flatter battery will normally take most of the charging currents but as the level of charge in the two batteries rises then small differences between the batteries will have an increasing influence on this ratio. When batteries get old it is not uncommon for a cell to become shorted out internally by lead paste that has fallen off the plates lying in the bottom of the cell box. This results in the nominal battery voltage being nearer 10 than 12 volts. When this happens the 10 volt battery will quite quickly discharge the "good" 12 volt battery resulting in only 5 effective cells working in the 10 volt battery. Hence, if you want more capacity its always better to SERIES connect two big 6 volt batteries. If you want to parallel connect 12 volt batteries then two batteries that are similar in all respects is the second best option. I have two 6 volts in series. Link to comment Share on other sites More sharing options...
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