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Long Involved Question on Different Batteries & Amp Hour Ratings

I camp with a motorcycle pulling a tent trailer. I also need a battery powered CPAP (sleep apnea pump) at night. My motorcycle has a 18 amp-hour battery, I installed a 35 amp-hour battery in my trailer to run the pump.
While traveling, the bike charges the trailer battery, but what happens when I stop for several nights in a row? I disconnect the trailer and ride the bike around during the day.
I have drained the 35 ah battery in about 3 1/2 nights while parked. I understand charging voltage but I don’t have a good grasp of the amp-hour situation. If I connect the two batteries directly to each other while parked, will I gain amps transfer so I can run my pump w/o having the bike motor running to charge the trailer battery? I don’t want to drain the bike battery so I can’t start it.
Grateful for any insight or useful suggestions. Thanks in advance.


I don’t know the answer to your specific question. Is there electricity where you camp so you could plug in a charger? Could you carry the trailer battery with you and charge it while driving around without the trailer?

Another idea: have an extra cycle battery that you can swap in and out, keeping both charged up as much as possible.

Better idea: a solar powered charger.

If you connect two batteries + to + and - to -, you will increase your running time. For your application, you should be using a deep cycle battery, not another motorcycle battery. Is the battery that is in your trailer something that came with it or did you add it.

If you added it, and you have the room for a car sized battery, I would recommend an Optima Marine battery. They are not cheap, but it should give you several extra days, possibly up to 10 days.

You could also look for a small generator to recharge the battery every couple of days. There used to be a generator made by Honda that was good for about 250 watts, very quiet and small and didn’t use much gas. It would charge your present battery in about 2 hours.

If you connect the batteries + to + and - to -, the Amp-Hours add, so 35 + 18 = 53 AH. Thus you would get another 1.5 nights or so. However, that would of course drain the motorcycle battery along with the trailer battery. As others have suggested, it seems you need either more/bigger batteries, or find a way to charge the trailer battery while you’re in camp.

Spring the extra bucks for a campsite with electricity, it could be tax deductible, look into a 12 volt solar charger and a separate deep cell battery, or try the ranger station to put an extra battery on charge with a battery tender during the day, are my first few thoughts. If you are using an inverter consider trying to find a 12 volt apnea machine as I would think it would result in less battery drain.

I really don’t know the technical issues you raise but I think what you want to use is a deep cycle marine battery such as for electric trolling motors. They are designed for a longer cycle and frequent charge/discharge cycles.

You nailed it Bing.

You don’t want to use an engine starting battery for continuous power. Not designed for it and isn’t good for battery life. A deep cycle battery is perfect for this application.


Way up nort here we call it a deep cell, but deep cycle seems to be da same ting, ya?

Nah! Yah gotch yur wet cell or da dry cell battery. No such ting as a deep cell battery.


Sounds like your pump uses 10 amp/hours a night…What you take out, you must put back…A Deep Cycle battery is a must as Tester pointed out. A group 24 size will deliver about 60 AH and a group 27 size will provide 100 AH. Lead-Acid storage batteries will last MUCH longer if you limit the discharge to about 50% of the batteries capacity…Asking your motorcycle to charge a big, discharged battery is flirting with disaster as the alternator in the bike was not designed for that type of service…It may tolerate it for a while, but I would find another way to charge your pump battery…

Okay, A couple of you have made points I didn’t include in my original post. The battery I installed in the trailer is a sealed AGM deep cycle; I am aware that more than 50% discharge will shorten the battery’s life span. I’m dry camping without plug-in available, therefore the battery in the trailer without availability of a Battery Tender. Size and weight are considerations so the suggested larger group batteries don’t fit my plans. I was wondering if I could extend the trailer battery life somehow by attaching to the bike battery when parked. There is no problem with a single night parked, but multiple nights without recharging does create the issues I’ve raised. I have the proper sine wave inverter for my pump. I was looking to work with what I had on hand (I’m on the road right now and have a 7 day stop w/o electricity coming up that wasn’t originally planned for). Looks like a solar charger becomes my option at this point.

Thanks all for the responses,


You sound like you understand all the technical issues. I’ll just mention that your sine wave inverter is going to lose some energy (nothing is 100% efficient), so IF there is a DC-powered CPAP available, you could gain a little operating time by buying one of those. That would also save your hauling the inverter. But FAIK the inverter is 90% efficient, and the gain would be minimal.

Good luck, and happy camping.

Thanks, I am slightly knowledgeable, I just didn’t have a good practical grasp of how amps work in the battery equation. Volts I understand, watts I understand, so amps has to fit in there somehow. Like I wrote above, I’m beginning to think that I need to pick up a small solar charger.


If you decide to go solar, this would be about the smallest unit you could get by with.

I looked at the smaller 5 watt panel but doing the math, it would only add about a day or so to you trip. It would only restore about 2.7 amp-hours per day, you are using about 10.

At about 1.2 amps per hour and about 6.5 hours of useful charging sunlight, it won’t allow you to go indefinitely, but it will make a 7 day or longer trip possible. Look at the size of it though.

You can recharge your battery using the motorcycle’s alternator, but that will take a up to an hour and use some fuel, but you would do that once every three days.

If you did run your motorcycle battery dead, can you kick start it?

FYI for those not familiar with sleep apnea, CPAP machines are quite expensive. There are only two models that I know of that can run directly off 12vdc, and they consume about 6 amp-hours per night. They also have limitations that some users cannot live with. Most of these are obtained through insurance and there rules there, you can’t order just any kind of machine at any time.

A solar panel big enough to charge this battery in a reasonable time would be too big to take with you.

You will need at least a 30 watt (2 amp) panel to make a meaningful difference in your set-up. Finding the storage space for one in your trailer might be difficult. Two 15 watt panels might have dimensions more suitable for your needs. Also, flexible panels exist, maybe even some you can roll up…To work well, they must be placed in direct sunlight with no shadows covering the panel.

By connecting the bikes starting battery and your camp battery in parallel, you CAN extend the operating time of your CPAC machine at the expense of discharging the bike battery and shortening it’s life…You can also expect the bikes alternator to overheat as it was not designed to charge a large battery pack. Motorcycles tend to have rudimentary charging systems not designed for this sort of thing…

The relationship between Volts, Amps, and Resistance is explained in Ohm’s Law…If you know two values, you can easily calculate the third…
Volts X Amps = Watts…

A 30 watt panel would fully recharge his battery everyday. To just extend a trip, he can get by with a 15 watt panel. A 15 watt panel will restore about 8 amp-hours per day, he is using 10 amp-hours per night. At a 2 amp-hour deficit per day, he could go about 15 days before he had to return to homebase and recharge his portable battery.

I did not see any flexible panels for 12 volt systems, they were for recharging a pair of AA batteries only.

eBay is your Friend…Here is a 15 watt (one amp) flexible panel. There are MANY more, up to 150 watts that roll up…

I would still go for 30 watts…During all these energy conversions, you lose 10%… Pretty soon, you come up short…

Good find.

I’ve had similar issues rustic camping with the bass boat. Charging a trolling motor battery after 1-2 days use. What worked for me, we stopped at nearest gas station and asked if they would put it on their charger overnight and how much? No problem and charged me $2.00. You might try the same. Drop off in am, pick up on way back to campground.