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Thread: What size deep cycle battery?

  1. #1
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    Red face What size deep cycle battery?

    Please forgive this post. The answer I seek seems to be embedded too deeply in previous posts to find 'cause I've been searching the forum for over an hour without luck.

    I need a new deep cycle battery for the Skamper pop-top I recently acquired and am fixing up. What amperage should I get? I'm going to change out the three interior area lights to LED and would like to power a CD player. I don't have a frig but may want to add one sometime in the future. My heater and burners run on propane so no draw there. There are clearance lights that currently come on whenever the interior lights are turned on when plugged into 120V but I'm going to separate those circuits. I'll typically only be out for 2-3 days at a time. I do plan to add a solar recharge set-up sometime in the future.

    What size battery do I need?

    Thanks for the help!

  2. #2
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    Quote Originally Posted by Stereo View Post
    Please forgive this post. The answer I seek seems to be embedded too deeply in previous posts to find 'cause I've been searching the forum for over an hour without luck.

    I need a new deep cycle battery for the Skamper pop-top I recently acquired and am fixing up. What amperage should I get? I'm going to change out the three interior area lights to LED and would like to power a CD player. I don't have a frig but may want to add one sometime in the future. My heater and burners run on propane so no draw there. There are clearance lights that currently come on whenever the interior lights are turned on when plugged into 120V but I'm going to separate those circuits. I'll typically only be out for 2-3 days at a time. I do plan to add a solar recharge set-up sometime in the future.

    What size battery do I need?

    Thanks for the help!
    My camper van has a 100ah flooded -deep cycle- aux battery under the hood. It charges via split-charge relay when the engine is running, and via small generator and battery charger when camped. It's old and it's been rode hard and put away wet (i.e., drained well below 50% and then not properly recharged in a timely fashion).

    Running just an exhaust fan on the overhead vent, an oscillating fan inside, one or two lights (1141s, not LEDs) and recharging my netbook and some AA and AAA batteries it goes a good three days before it's pretty much dead. It did about twice that when it was new 2 years ago so the frequent abuse has reduced it's capacity by nearly 50%, so I suppose now it's a 50ah battery instead of a 100ah battery.

    I plan to replace the charger with a good 2-stage charger (Samlex SEC-1215a) and replace the battery with another new 100ah cheapo flooded deep cycle.

    I buy cheap batteries because I know I'm going to abuse them, but I do NOT buy RV/Marine batteries for aux use. I buy deep cycle batteries.

    I haven't bothered with solar because I park in the shade when I can, and when I can't I have a surplus parachute that covers the entire vehicle and serves as a good sized awning on all 4 sides.

    Batteries are basically "watts per pound" so whatever size battery fits - Group 27, Group 31, etc. The battery I have now is basically like this:

    http://www.cloudelectric.com/product_p/ba-27dc36.htm

    But without the label. There is a battery shop in my area that often gets their hands on batches of "no label" and/or "scratch and dent" batteries. I paid $45 for the one I have now.
    ...
    ...
    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  3. #3
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    Quote Originally Posted by Stereo View Post
    What size battery do I need?

    Thanks for the help!
    Go visit this website for some ideas on sizing. Dan did a small camper refit for a fishing camper/pop-top... they're just up the road from you in Boulder, they're in Fort Collins.
    "Good roads lead to bad fishing".
    Eric Wight, Maine game warden

  4. #4
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    Default Wire size? Venting? Grounding?

    Thank you for all the information. Great link for a basic understanding of power and solar! The article led to more questions beyond what size battery to get.

    My old Skamper has a built-in inverter. The former owner connected the deep cycle battery to the inverter panel with stranded lamp wire. Clearly, that's not appropriate, but other than saying to use "thick and flexible" wire for that connection, the article did not specify size. What gauge wire should I use?

    One illustration of wiring off or to a battery suggested the battery should be grounded. Is that necessary? If mine's inside the camper, how might I ground it?

    Other articles talk about the need to vent the battery. Is that necessary? I'm not keen on punching more holes in the camper but I also want to be safe. I might be able to put it outside the camper and feed the wiring inside, but others have talked about that making the battery more vulnerable to theft.

  5. #5
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    Quote Originally Posted by Stereo View Post
    My old Skamper has a built-in inverter. The former owner connected the deep cycle battery to the inverter panel with stranded lamp wire. Clearly, that's not appropriate, but other than saying to use "thick and flexible" wire for that connection, the article did not specify size. What gauge wire should I use?
    Inverter, or converter? Inverter changes 12v DC to 120v AC. Converter changes 120v AC to 12v DC (and also usually does a bit of battery charging).

    The wire size is determined by the amount of load on the wire (in amps), and the length of the wire. Wire has a rating of the maximum amps that it can carry, but also voltage drops over distance, so a long run of wire might need to go up a size or two in order to carry the amps without dropping the volts too much.

    Assuming we're actually talking about an inverter, the first step is to figure out how much it draws from the battery. Let's say it's a 1000w inverter that can briefly handle a surge load up to 1500w. We need wire that can handle the larger load - 1500w.

    Now watts is watts. The inverter needs to supply 1500w @ 120v on the output, and so it need to draw 1500w @ 12v on the input. Now we do the most basic of electrical math: watts / volts = amps

    1500w / 12v = 125a

    So we need wire from battery to inverter which can handle a 125a load. Now we consult a wire gauge chart:

    http://www.powerstream.com/Wire_Size.htm

    We're doing power transmission ("chassis wiring" does NOT mean vehicle chassis - it means machine chassis - i.e., in an enclosed space), so we look in that column and we see that we need wire size 0, which can handle up to 150a. Now we know what size of "big, fat" wire we need to feed that inverter. That size would be fine for a short run say up to 6'. For a longer run, we might have to go up to a larger size to keep the voltage from dropping too much.


    (Note that this is 10 times the size of wire we'll need on the output side of the inverter because the output is at 120v. Doing the same math formula:

    1500w / 120v = 12.5a

    We only need wire on the output side capable of handling 12.5a. 14 ga. wire and a 15a breaker or fuse would handle that nicely.)


    One illustration of wiring off or to a battery suggested the battery should be grounded. Is that necessary? If mine's inside the camper, how might I ground it?
    Vehicles aren't actually grounded - they aren't connected to the planet - nor should they be unless they are something like a truck mounted generator being used to power a building, or like a boat or RV being fed from shore power.

    In a vehicle, "ground" usually means "to connect to the frame". You should have both a positive and a negative wire from battery to inverter, and another pair from charger to battery. But yes, it's best if there is -also- a wire from battery to frame so that if any positive wires get loose and happen to touch the frame, it will provide a path back so that the fuse can blow for safety.



    Other articles talk about the need to vent the battery. Is that necessary? I'm not keen on punching more holes in the camper but I also want to be safe. I might be able to put it outside the camper and feed the wiring inside, but others have talked about that making the battery more vulnerable to theft.
    If it's a flooded battery then yes, it's absolutely required to be vented. Flooded batteries offgas hydrogen when they are being charged. I don't know if you've ever seen hydrogen burn...but it burns so fast that it fits the description of "explosion". And it lights up pretty easy too.

    You DO NOT want hydrogen venting into your living space.

    Sealed batteries, which might be a flooded (liquid electrolyte), AGM (electrolyte absorbed into fiberglass mats), or Gel (jellied electrolyte) don't offgas hydrogen - unless they are overcharged and the sealing valve pops to relieve pressure. Then they are no different and they will vent hydrogen when being charged. Technically, it's best to have them in a vented space as well - though often people don't do that. They can get away with it as long as they make certain they don't overcharge the battery and pop the valve.
    ...
    ...
    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  6. #6
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    My parents and I just use the regular deep-cycle batteries off the rack at Walmart. They're cost effective and very good with their warrantees.

    If your furnace is an RV type (built-in with vents to the outside), it has an electric blower that uses a fair bit of electrical current.

    Older and simpler propane fridges can run on propane without any electricity. Many more modern ones have electronic circuitry and air circulation that uses electrical current along with the propane (LED lights on the front panel are usually a give-away). These electronically-controlled fridges make it tough to maintain battery charge--they're always drawing current.

    My parents have an isolator connected between the truck battery and camper battery. It connects the two when the engine's running to charge the camper battery and then disconnects them when the engine's stopped. That way the camper electrical use doesn't affect the ability to start the engine.

    My camper and truck electrical systems are completely independent and there's a small (10-watt) solar charger on the camper. I don't have a fridge or furnace so the camper draw is pretty low. I like the solar-charge because it keeps the camper battery topped-off while it's parked outside. My parents have to hook up a separate charger because driving often doesn't charge the camper battery all the way and then it gradually discharges while it's parked.

    If you have a converter, you likely have a battery compartment that's vented to the outside. I have also seen setups that have the camper battery stored between the bed and the camper--that's a vented location. Mine is stored inside the under-counter cabinets and there is a vent in the back camper wall to vent the cabinet area.

    Wow--great write-up, dwh

    Eric
    Last edited by tanglefoot; 05-22-2011 at 05:09 PM.
    '85 4Runner & Four-Seasons Magnum pop-up

  7. #7
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    FANTASTIC information dwh. Not only did I get my answer, I got a thorough education too. You're right. It's a converter, not an inverter. The faceplate was partially painted over so I could only read "...verter" and now I know what's what. I'm fully versed in 120/240 house wiring but not 12v.

    Thanks, Tanglewood, for the reminder about the heater fan. I forgot about that electrical feature. I'll definitely add it to my calculations. I definitely want to hook up a solar charger too so any details you care to share would be much appreciated.

    I installed my Ride Rite air bags today and will post my experience with pics later.

    THANK YOU.

  8. #8
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    Older converters are basically just constant voltage power supplies which can hold their rated voltage up to a certain load of amps. So say the converter puts out 12.6v 30a - then it will hold 12.6v at any load up to 30a - a load higher than 30a would start to pull the voltage down if the unit didn't overload and shut down.

    Pretty similar to a regular old automotive battery charger. As long as the converter holds the bus voltage to 12.6v, then if the battery is below that voltage, it will slowly absorb power until it reaches equilibrium with the bus voltage. That's why they are often referred to as "converter/chargers".

    So the converter not only supplies power to whatever 12v loads are connected, it also slowly recharges the battery up to 12.6v.


    What you likely have is an old MagneTek power center. The 120v shore power comes in to it, and it passes that through to the 120v outlets in the camper. It also taps off the 120v to feed a built-in converter to supply 12v.

    The problem with those old converters is that 12.6v isn't really enough for a modern battery. Most modern batteries need to be pushed up above 14v and held there a while to be truly fully charged. A smart charger will do that, and then once the battery stops absorbing power it will drop back to a "float" voltage, which these days is usually between 13.2v and 13.6v depending on the charger. The older converters were by no means smart.

    The second problem, is that that old converter probably has a pretty low amperage rating. I.e., it might only supply 9a to the 12v bus and that's not much. The old Shumacher battery charger which is hard wired in to my camper puts out 12.8v and up to 10a and while it will eventually charge my battery up to 12.8v - it takes around 12 hours if the battery is really low.

    That is why I'll be replacing it with a modern multi-stage charger. Not only will it still function to supply 15a to the bus for loads (I don't have many), but it will do a much better job of charging the battery and my generator run-time should be half what it is now.


    If you add solar, then a decent solar charge controller, such a Morningstar Sunsaver, will be a multi-stage charger and will get the battery properly charged up to modern voltage specs.

    IF there is enough watts of solar panel and IF there is good direct sun on the panel.

    In that case, you could still use the ol' MagneTek if 9a (or whatever its rating is) is enough to supply your loads. It will only get the battery up to 12.6v, but the solar will override that when there is enough sun and push the battery the rest of the way up.
    ...
    ...
    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  9. #9
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    For me a maintenance free battey that was leak proof was needed. My house battery is in the front of the truck bed between the camper and the truck bed, complete PITA to get to. I went with an Optima, it has met my needs.
    Crew dog, Dust Junkies Racing, Three time Baja 1000 Champions, Class 1700 (Jeepspeed)

  10. #10
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    Default Still fuzzy.

    DWH: You are close to dead-on, only my output is even lower. Tonight, with a flashlight, I was able to read more through the paint. The converter is a Brand ?, Model CS506 with output of 12.5 VDC and only 6 amp.

    I understand that the converter converts 120V to 12V to power my 12V stuff. And I understand your explanation about the output from the converter not being enough to fully charge a modern-day battery. But I don't yet understand how my load gets supplied. If the converter only puts out 6 amp, and according to the bus bar, the converter uses 8 amps, there would not be enough output to power andything else.

    Original loads appear to be Converter - 8A and Clearance lights - 8A. I think a previous owner did some retrofitting and added a water pump, unknown amps, and 3 cabin lights with 1141 bulbs, 12V 1.44 amps each. My memory now tells me I don't have a blower in the heater (and I don't see any fan blades). If I had a frig, the panel claims it would draw 15A but I don't know if that's typical of today's frigs. I could also use propane for the frig. I'd also like to run a radio/CD or a laptop.

    I'm trying to understand the wiring. The bus bar has two sides, bridged at some locations by fuses. The old defunct battery was wired to the right side and the ground with lamp wire. The wires from the converter come in on the left and I'm assuming the other wires on the left - that aren't going to the ground - are going out to the stuff they power. There is one wire that connects the left and right sides. Does power from the battery come in on the right, pass to the left side, then power the stuff from that side of the bus directly?

    Bus bar.jpg

    ALSO, please double check me on calculating battery size. If I'm figuring it right, if I were to get a 105A-hrs battery, limit it's rundown to 50%, i.e. 52.5 amp-hours, I could run 1 light (1.44 amps) and 1 CD player (13W divided by 12V = 1.08 amps) for almost 21 hours (52.5 divided by 2.52 amps). Am I doing the math correctly?

    On the load calculation, I'm assuming the converter doesn't pull amps when I'm running off the battery. Am I right?

    Based on the wiring chart at your link, do I need to attach the battery to the bus with 0-size wire? Is that different than 0-sized wire for 120V systems, 'cause that would be HUGE.

    Thanks for your help and patience as I try to understand the 12V system.

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