If you`re going to tandem another battery with the LB you`ve got, it needs to be the same type, size and ideally age, according to general info. If you mix batteries then the weaker one (there will always be one) will pull the better one down to its level (bit like life really

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Chargers that will do sealed type LBs as well as normal lead acid have a selector switch to achieve the correct charging, so you can`t mix two different types and expect the charger to work properly.
You don`t need specific selaed leisure batteries, normal lead acid starter types work perfectly well for most people, and you can buy two 70 AH batts for around what you`ve spent on that LB, that`s why people stick with standard type batts.
Keep it real.
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