[bronsonj]

That lawnmower battery is not helping. It's so out of it's league in terms on sustained high current draw. A large car battery can give 400 or more amps for 5 or 10 seconds, while you start your car. But it cannot do 100 amps continuous drain for very long. A lawnmower battery is just not a complication to add to that pack really.

You need another 6v battery. The issue is if you have five 6v batteries that have 60% life left and one with 100% then when you charge them as a group the other 5 will be undercharged and the new one will be overcharged. In very short time you'll end up with six batteries that are all at 55% life left because of that.

The correct solution is to get a new set of batteries. You can get 6V or 8V golf cart batteries. The cheapest place I've found for new batteries is Sams Club. Not all Golf Cart batteries are the same but if you're looking for the cheapest way Sams Club is it. The batteries sometimes are on sale. You need to bring in cores but Sams doesn't care if the core is out of a car or emergency exit sign... At least mine didn't care. As long as it says Lead Acid somewhere on the battery they'll take it. Heck, I turned in a Power Wheels battery as a core! Otherwise it's $20 or $25 more if I remember correctly. The Sams near me has 6v golf cart batteries right now for $108, not on sale. I think the once a year sale is like $20 off each battery.

There are a few 12v golf cart batteries. Those really aren't very good and will generally need replaced sooner and give you less range and less useful life compared to 6v or 8v batteries. But still you could buy four 12V golf cart batteries likely cheaper than six 8v or 6v batteries. I have not priced 12v golf cart batteries.

Whatever you do, don't buy Marine "Deep Cycle" batteries or regular car starting batteries. It's not designed for the abuse golf cart batteries are made for and they will die a short painful death.