Battery tender... 5 watt for $50, or 15 watt for $150

[TygerCub]

Okay guys, I have a blond question. Mom, my aunt & I were wandering through Northern Tool today and there was a 5 watt batter tender for $100. http://www.northerntool.com/webapp/w...0124_200140124

But along the back wall, there was a much larger 15 watt battery tender for $150.
http://www.northerntool.com/webapp/w...8565_200328565

I'm thinking that getting 3 times the power for $50 more is a good thing. Seemed like the logical thing to do.

Then I got to thinking some more - fanned the smoke from around my head - and wondered if I shouldn't just get the smaller one for now since it is purely for crank battery maintenance and just save the extra $50 for a larger (200 watt larger) panel later?

Just how much power does one need to maintain a crank battery for the bus? Would the 5 watt do it, or would I really need the 15? Could the power from the 15 be fed to both the crank and the "house" batteries? Is there a way to split that power input?

And if the 5 watt is good enough, would something like this (which is half again the price) work just as well???
http://www.northerntool.com/webapp/w...5091_200305091


There are two batteries in BOB's compartment and they are hooked together. Don't know if it's in series or parallel (cable from bus to positive battery #1 to positive battery #2, & negative battery #2 to negative battery #1, then back to the bus). Does it matter how the batteries are hooked together for this application?

My brain hurts and something smells burnt

[Nachtwulf]

Tygercub,
You have opened a whole new "can of worms" as they say, about numerous and sundry different subjects. Let me see if I can sort out some specifics.
First: What is the difference between a "battery maintainer" and a "battery charger"? A battery charger is a simple device (of several possible types, solar, electronic, generator, etc.) which puts out a slightly higher voltage than the rating of the battery in order to force an electrochemical reaction within the battery, thereby "restoring" the charge. Since charging rates are variable, some adverse reactions are possible within the battery, including "sulfation" which will reduce the output capacity of the battery and can ruin it. However, a battery maintainer is designed to put out a measured pulse of direct current which is of a frequency found to minimize the sulfation of the battery plates. This is why the "maintainer" costs so much more than a standard charger, such as the 5 watt solar charger. Normally, anything which keeps a battery at a fully charged condition will do a fairly good job of preventing sulfation.
Second: If you are regularly discharging and then fully recharging your starting and/or "house" batteries, there will be minimal chance for sulfation. However, extended discharge with long intervals before fully recharging WILL cause sulfation and premature battery failure. Your crank batteries should get recharged every time you run BOB for any length of time. If you are not running that often, 5 watts will probably be enough to keep them fully charged. If you wish, it is possible through use of a simple battery isolator to keep both sets of batteries "maintained" at full charge. http://www.northerntool.com/webapp/w...3388_200343388
Third: Your batteries are hooked up in a parallel configuration (positive to positive and negative to negative).
I know I haven't fully answered all your questions, but maybe it will give you some info to digest for your decision.
I hope this helps,
Lee

[TygerCub]

Thanks for the info Nachwulf! Now let me see if I understand what you just said

For now, the bus only has the 2 crank batteries - no house batteries yet. it sits for long periods of time without running - actually it can't run yet at all This means the crank batteries are not being recharged on a regular basis, because even when the bus did run, it was only for very short periods of time to test whatever we're working on at that moment.

With the info you've provided, I would believe the 5 watt panel will be sufficient for my crank battery needs.

But what I'm still fuzzy on is if I should get the "maintainer" or the "charger" to keep the battery while it's sitting idle in the RV lot. Even when the bus is completed and on the road, there will be long periods of being idle (up to several months if I'm lucky). So for that type of setup, would I get the "maintainer"?

[Redbear]

A disconnect switch may be in order. Assuming you did not arrive at your parking place in a blizzard or extended downpour, where all the wipers, lights, heaters sucked up the alternator output and left nothing for battery charging, you should arrive at your destination with fully charged batteries. Throw the disconnect, and there will be no leakage on the starting batteries to draw them down. Make sure the cases are clean so that there is no grime or acid path connecting the terminals to trickle power away. A fully-charged battery will last for some time with the wiring disconnected, but a discharged battery should be recharged ASAP.

If it were me, I would use the smaller one. I would worry about the higher capacity one boiling water out of the batteries if you don't watch the levels. But then, I have only used chargers and voltage-regulated supplies, and never the high-frequency maintainers.

When I lived in the mountains, and the temps were in the minus Fahrenheit at night, I used a 1-amp charger to warm the battery, together with a lower hose heater for the antifreeze and a stick-on magnet heater secured to the tranny pan. A timer turned them on 4 hours before my commute. With the low charge rate, I never had a problem with either discharge or over-charge.

[frank-id]

Most charging devices are rated in amps, not watts. Watts is a words that expresses total electrical energy. A 5 watt charger would be .4 amps at 12 volts. Huh? Redbear uses the method I recommend, with a small 10-15 amp charger connected to a 24 hour timer. The timer is set to charge about 4 hours every 12 hours. This system will not overcharge and the batteries will be ready. With 24volt battery systems, each battery can be charged singularly and a 24 volt charger not required. A voltage reading of a 12 volt battery that indicates 12.0 volts is 75% discharged. A fully charged battery will show 12.6 as each cell will create 2.1 volts per cell.. Frank

[Redbear]

A fully charged battery will show 12.6 as each cell will create 2.1 volts per cell.. Frank

Voltage tests to determine state-of-charge are generally done after being disconnected from both charger and load for 24 hours to stabilize.
A battery might show 14 or 15 volts 30 seconds after being disconnected from the charger, regardless of the true state-of-charge.

The best way is getting and learning to use a hydrometer to measure the specific gravity of the acid solution. With a sealed battery (like AGM), a voltage reading will have to suffice.

[Nachtwulf]

Originally Posted by TygerCub

Thanks for the info Nachwulf! Now let me see if I understand what you just said

For now, the bus only has the 2 crank batteries - no house batteries yet. it sits for long periods of time without running - actually it can't run yet at all This means the crank batteries are not being recharged on a regular basis, because even when the bus did run, it was only for very short periods of time to test whatever we're working on at that moment.

With the info you've provided, I would believe the 5 watt panel will be sufficient for my crank battery needs.

But what I'm still fuzzy on is if I should get the "maintainer" or the "charger" to keep the battery while it's sitting idle in the RV lot. Even when the bus is completed and on the road, there will be long periods of being idle (up to several months if I'm lucky). So for that type of setup, would I get the "maintainer"?

Hi TygerCub,
Sorry if I caused more confusion. The short answer to your question is that the maintainer is probably better for your situation, and the larger one is probably the better buy. Why? Well, I am not trying to get you to spend more to stimulate the economy. But with any solar charger (cheap kind) the stability of both voltage and amperage is uncertain at best. With the maintainer, you get better stability of voltage, current limiting where necessary, and the pulse technology to help stop plate sulfation. Since Watts equals Amps times Volts (W=A x V), then for the 15 Watt unit the maximum charge would never be more than about 1.1-1.2 Amps. This will not cause a problem with off-gassing, boiling off water, or any other evils of overcharging. And since BOB is not running currently, this should help keep those batteries fully charged even after testing and working on various bus systems. I certainly hope this helps, have a great week.
Lee