Dual Battery Setup Help

[dwh]

Thanks for the tips. I've been considering a pair of Odyssey PC1200s, if I can fit them in the factory location. Otherwise, it would probably be a combo of a DieHard Platinum P2 for the main, and a DieHard Platinum PM2 (marine) for the auxiliary.

Both the Odyssey and DieHard Platinum are AGM batteries, so I should be OK, right?

I'm not a big fan of engineering by the "should be" method. If ya gotta, then ya gotta, but better to avoid it if possible.

Looking at the Odyssey spec sheet I see a couple of things.

http://www.odysseybattery.com/autospecs.html

For one, the PC1200 is only 42 amp*hours (at the 20 hour rate). 42ah * 12v = 504 watt*hours. So you could run a 50w load for 10 hours, or a 100w load for 5 hours.

(More or less - the actual numbers vary as the load goes up or down. That battery is rated at 42ah at the "20 hour rate". So it would run a 25w load for 20 hours. A 100w load (faster drain) would lower the amp*hours number, so it would run for noticeably less than 5 hours.)

If you actually did that, the battery would be completely dead. That would be bad news. Generally, you want to discharge no more than 50% if you can for long battery life. So cut the runtimes in half. The truth is, 42ah is just not a whole lot of juice. You need to figure out your loads vs. run times. "A man's got to know his (battery's) limitations."

Down at the bottom of that page, to the right we see some important numbers. Standby charger voltage 13.5v - 13.8v. That would be the voltage range you would want from a portable charger or an RV converter/charger. Basically, that's the voltage range you want if you are going to feed a constant charge to the battery (float charge). Like an RV plugged into shore power and the converter/charger is supplying voltage 24/7.

Cyclic voltage 14.4v - 14.8v. That would be the MAX voltage range you would want from your alternator at highway RPM. It would be perfect if the charging system put out 13.5v - 13.8v at idle. Unfortunately, many alternators put out almost nothing at idle since they are designed to operate "in the zone" at a higher RPM.

Those are the normal voltage range specs for most modern "12v" FLA and AGM batteries (GELs often have different voltage specs). Older RV converter/chargers are usually lower than that. I looked at a 70's Winnebago that had a converter/charger that put out, IIRC, 12.2v. That would be too low to keep a modern battery topped up. My old Shumacher 10a charger puts out a constant 12.6v, which is almost 1v too low to fully charge a modern battery. (But that's okay. I get it close with that charger, and then let the truck's alternator finish it off. Also, I don't have to worry about overcharging my battery if I forget and leave that Shumacher turned on.)

So what voltage range does your alternator put out? You ought to know the answer to that question.

Another VERY important thing to note at the bottom of that page is the Typical deep-cycle life. Take it down to 80% depth of discharge (20% remaining) and it's good for 400 cycles before it's toast. If you don't go below 50% it'll probably last a couple of thousand cycles. Take it down to fully dead and who knows...a dozen cycles? Two? Maybe more, maybe less.

(Actually, it won't usually be completely ruined. Normally, what happens over time is that the battery can only absorb less and less power. So after too much deep drain abuse, your 42ah battery might only hold 5ah - which I would call "toast".)

Also note that on the charging voltages it says, "no current limit required". That's very good, as it means that you can recharge those batteries with high amperage. You could recharge that 42ah battery with a large alternator. For example the 100a alternator in my truck could provide an amperage 250% of the battery capacity and those batteries should handle it no problem.


The Platinum PM-2 has a 68ah capacity. Quite a bit more than the PC1200. So if it will fit, and the price is close, I'd go with more amp*hours. I don't find any specs on charging voltages. Sears probably has an owner's manual for that battery that will almost certainly have those numbers. Very likely they'll be pretty much identical to the Odysseys.


Both of these batteries have cranking amps ratings, which almost always means that they have thin (starting) or medium (rv/marine) plate thickness. True deep cycle batteries have thick plates and almost never have a cranking amps rating. If you keep looking, you might find a "true deep cycle" that will fit and give even more amp*hours capacity.

 

[alexrex20]

thanks for the excellent response, and in terms even I can comprehend!

i bring up the "should be" scenario because i don't plan on doing any heavy battery pulls. i don't have a fridge, i don't have a bumpin' audio system. to be honest, the only reason i want to do dual batteries is to have that backup for if/when my battery and/or alternator dies, or i leave the key in the ON position for too long. my truck has a factory timeout that cuts everything off 5min after the engine is shut off. the only way to enable anything electronic is to put the key back into the ignition and turn it to ON. it kinda makes it fool-proof in terms of accidentally leaving something on.

i like the idea of having the 2nd battery built in, and the ease of just flipping a switch to jump start myself. the only high current draw "accessory" on my vehicle is the 8klb winch, and it hardly sees any use.

for all intents and purposes, i really feel a pair of auto/marine batteries (with thin or medium plates) will be more than sufficient for my needs.

 

[dwh]

or i leave the key in the ON position for too long

One thing to keep in mind with a solenoid setup that if the key is on, the batteries are normally tied together - so if you aren't careful you could end up running them both flat.

Otherwise it sounds like you have a pretty good grip on what you're doing.

Cheers.

 

[dwh]

i like the idea of having the 2nd battery built in, and the ease of just flipping a switch to jump start myself

Oh, and that may or may not work. On my truck it wouldn't with regular battery cables. I had to install thicker battery cables before it would jump start through the solenoid (but even with normal battery cables I could still jump it using jumper cables).

 

[alexrex20]

One thing to keep in mind with a solenoid setup that if the key is on, the batteries are normally tied together - so if you aren't careful you could end up running them both flat.

I've already purchased the Painless dual battery control setup, so I guess it'll have to do. It uses a 3-way switch, allowing you to do 3 things with the auxiliary battery. It only affects whether the auxiliary battery is connected to the main battery or not; either it is (1) connected all the time, (2) via the ignition switch or (3) not at all.

I plan on leaving the auxiliary battery isolated most of the time, and while I'm cruising on the highway, I'll flip it to be connected only while in the ON position (mode #2). That way, it will again isolate both batteries once the ignition is turned OFF.

Nevertheless, I have this thread bookmarked.

 

[alexrex20]

I've just acquired one of these beauties:

http://www.google.com/m/url?client=...UQFjAB&usg=AFQjCNF16OtL-DZ-P8xoQFK7WuQi1C6Acg

It's an Enersys/Hawker HASP-FT military battery, rated 240min reserve capacity.

It's a gel battery(?) so can I just hook it up as the auxiliary battery, as though it were a normal automotive battery? My alternator outputs 14.2-14.7V, if that means anything.

 

[dwh]

I've just acquired one of these beauties:

http://www.google.com/m/url?client=...UQFjAB&usg=AFQjCNF16OtL-DZ-P8xoQFK7WuQi1C6Acg

It's an Enersys/Hawker HASP-FT military battery, rated 240min reserve capacity.

It's a gel battery(?) so can I just hook it up as the auxiliary battery, as though it were a normal automotive battery? My alternator outputs 14.2-14.7V, if that means anything.

According to the datasheet, it's not GEL, it's AGM (that's good):

http://www.marcorsyscom.usmc.mil/sites/pmeps/DOCUMENTS/HawkerArmasafe.pdf

According to a different data sheet, it's a 120ah@C20 rate battery:

http://www.hawkeraplus.com/pdf/a+_data_sheet_supplement.pdf

One data sheet lists 360 cycles - 120 deep. The other just says 350 cycles.


Looks like a really nice battery. Only thing that bothers me is the shelf life...makes me wonder - "how long did it sit around before you bought it?"

As for charging - that second data sheet says to charge at between 14.0-15.0 volts. That's right in the normal AGM/FLA range and just right for your alternator, so no worries there.

 

[alexrex20]

The battery was free! My buddy got a lot of them from his master mechanic (military) as they had been run down and swapped out. As far as I know, they only recently came out of a vehicle.

It's reading 5.88V on my multimeter and I have it hooked up to my little 6A battery charger. I expect it to take a day (or two) to get it fully charged. Are there any guidelines I should follow to know when it's fully charged? [EDIT: I found the manufacturer's recommended procedures for a deep discharge condition. They recommend constant charge at 5A, and it may take 36hrs or more.]

I'm glad to hear that it's an AGM battery. Thanks for posting those PDFs. I was having a hard time researching and then posting here on ExPo, from my phone. These "smart phones" are only as smart as their users!


EDIT: I've read somewhere that marine (or more specifically deep cycle) batteries aren't designed to be continually recharged. Is this true? Am I not supposed to keep the Hawker auxiliary battery continually in the charging loop? I read back in the thread and you mentioned the best way to prolong battery life is to always keep it fully charged...

 

[wrcsixeight]

Some chargers will not charge a battery that is down to 5.88 volts. You must use a pair of jumper cables to a good battery and then charge both batteries.

When the drained battery can hold 12 volts on it's own, the charger should be able to take over. If the battery gets hot, stop charging it and give up on the battery.

6 amps is a little light, it might not be able to reverse the sulfation that higher amps could. If the battery has sat for a long time the sulfates might have hardened beyond any charger's ability to dissolve back within the electrolyte. At 5.88 volts the battery could is so dead that a 6 amp charger will need 3 to 4 days to get it to full charge, depending on the size of the battery.

You can keep a deep cycle battery on a float charger at no more than 13.6 volts. Flooded batteries can develop electrolyte stratification if kept for a long time at 13.6, but 14.4+ volts will bubble the battery, mixing the layers.

 

[alexrex20]

After 24hrs it's up to 12.3V. My charger is pretty pathetic because it's only outputting 12.56V. Once battery OCV is up to that 12.56, I'm going to let it sit for a few hours to see if it will hold the charge.

I need to find a more powerful charger to borrow...

You can keep a deep cycle battery on a float charger at no more than 13.6 volts. Flooded batteries can develop electrolyte stratification if kept for a long time at 13.6, but 14.4+ volts will bubble the battery, mixing the layers.

Forgive the ignorance, but does this mean I shouldn't keep this battery in the charging loop for extended periods?

 

[wrcsixeight]

With a 6 amp charger, the battery voltage will rise as the battery charges. You can't just turn it on and expect 14+ volts.

With the alternator at high rpm, or a 40 amp + charger, you can get the battery voltage to rise higher much faster.

Your charger might be okay if it eventually drops the battery voltage to the 13.2 to 13.6 range, after it thinks it fully charged the battery.

A float charger like battery tender or batteryminder is designed to just keep them safely topped off. Harbor freight sometimes has a special on a similar product.

Just monitor the batterytemp and voltage with your current charger.

AGM batteries that have been abused by being brought down to 5.88 volts might develop a case of thermal runaway when being charged

 

[dwh]

The battery was free! My buddy got a lot of them from his master mechanic (military) as they had been run down and swapped out. As far as I know, they only recently came out of a vehicle.

Hate to say it, but there is a very high chance that it may be junk.

Lead-acid batteries make lead sulfate as a normal part of doing business. The problem is when that sulfate sits around it hardens into crystals. Once that happens, good bloody luck breaking up those crystals.

"Battery desulfators" claim to be able to do it with high-frequency pulses...basically vibrating the crystals into breaking up. No one knows if these things actually work...but a lot of people install them on the, "what the hell...can't hurt" theory.

(It's worth mentioning that the solar crowd has started noticing that desulfator's high-freq pulses seem to interfere with the proper operation of MPPT charge controllers, so the solar guys are starting to disconnect their desulfators if they have MPPT charge controllers.)

Personally, I've seen docs from one of the battery manufacturers (Concorde maybe...can't remember which one...) where they did tests on desulfators like back in the 80's or something and the desulfators DID break up some crystals. So in my opinion, they probably do work - at least a little.

The greater the buildup of hardened lead-sulfate crystals - the lower the capacity of the battery. If the buildup is thick enough, it can even short positive and negative plates together and then for sure the battery is junked. But even without it being shorted internally, it can still have such a low capacity that it's not worth bothering with.

You might very well get that battery up to a proper fully charged voltage - 13.2v-13.6v after fully charging up to 14.4v and then allowing to rest for 6 hours with no charge and no load before measuring. But even if it does come up to proper voltage, that still doesn't tell you how much capacity it has. That 120ah battery might only have 5ah capacity due to hardened internal lead-sulfate crystals.

If it's been rode hard and put away wet - AND/OR left sitting around uncharged - AND/OR drained down to 5v - then it DOES have hardened sulfate crystals. The question is how much.

(GOOD LORD MAN! FIVE VOLTS? THAT'S HORRIBLE!...seriously...I'm not joking...12v batteries are considered completely dead at 10v...5v would be like...practically a negative number. I'm surprised that battery hasn't imploded and become a Black Hole.)

It's reading 5.88V on my multimeter and I have it hooked up to my little 6A battery charger. I expect it to take a day (or two) to get it fully charged. Are there any guidelines I should follow to know when it's fully charged? [EDIT: I found the manufacturer's recommended procedures for a deep discharge condition. They recommend constant charge at 5A, and it may take 36hrs or more.]

Well, that would be with a constant current (amperage) charger.

You're probably using a constant voltage charger. A constant voltage charger keeps the voltage at a certain point (12.56 in the case of your charger). As the battery voltage rises there is less difference between the voltage of the battery, and the voltage of the charger, and so the amperage that flows toward the battery goes down. Finally, when the battery has reached the same voltage as the charger, there is no longer any voltage difference, and power doesn't flow either way. The battery and the charger are balanced against each other.

That's not exactly what they mean by a constant 5a charge.

A constant current charger will keep stepping up the voltage to stay ahead of whatever voltage the battery is at. It will make sure that there is always however much voltage difference is required so that the battery continues to absorb 5a (or whatever amperage the constant current charger is set to deliver.)

Think of it like downhill flow. With a constant voltage charger, power will flow from the charger to the battery until the battery reaches the same height as the charger - then the flow stops. With a constant current charger, the charger will keep raising its height to stay above the battery - the flow doesn't stop until the charger decides to stop it.

EDIT: I've read somewhere that marine (or more specifically deep cycle) batteries aren't designed to be continually recharged. Is this true? Am I not supposed to keep the Hawker auxiliary battery continually in the charging loop? I read back in the thread and you mentioned the best way to prolong battery life is to always keep it fully charged...

It depends on the charger.

With a flooded (wet) battery, you have the issue that wrc68 mentioned of stratification - basically the electrolyte separates into layers and needs to be stirred up. With a flooded battery that is usually done with a process known as "equalizing". Basically, you overcharge the battery at like 15v for a while to boil the electrolyte and stir it up. Afterward, you might have to add some water. Some chargers do the EQ for 15 minutes every 3 days, some do it for an hour once a week.

AGMs have the electrolyte absorbed into the fiberglass mat - so they don't stratify and almost all of them forbid equalizing. The main reason is not so much that the 15v overcharge will really hurt anything, but they are a sealed battery and the overcharge can build up enough pressure to cause the seal to pop. That's bad since it will allow hydrogen to escape just like a vented wet battery - but with a sealed battery you can't top off the water like you can with a wet flooded battery. So once you blow the seal, it's only a matter of time till that battery evaporates itself dry.

So if you have a charger that automatically does an EQ...and if you can't disable the EQ function - then you don't want to leave that charger on a sealed battery 24/7 - sooner or later you're gonna pop the seals.


If you have a constant voltage charger - like your 12.56v 6a charger, or my old Shumacher 12.6v 10a, or most RV converter/chargers - then you can leave it hooked up. The battery voltage will rise until the battery voltage and charger voltage are equal, and then power stops flowing - they both push with the same force (voltage) so they are balanced.

If you had a constant voltage charger that put out a voltage like 14.4v, then you would NOT leave that hooked up 24/7, since that is too high a voltage to hold the battery at all the time. You can take a battery up to 14.4v to get it full, but then you have to back off and let it come down to it's "normal, resting voltage" which is normally 13.2v-13.6v. (And there's the basic problem with our little 12.6v chargers - they aren't really high enough voltage to fully top off the battery.)

You can NOT leave a constant current (amperage) charger hooked up all the time, since it will just keep pumping the amps in at the pre-set rate until the battery melts. (Not really. Most constant current chargers have a timer so they only run for a certain amount of time. But you get my point.)


It's all about the voltage. Smart chargers are generally "3-stage". The first stage is "bulk" where some of them run as constant current chargers until the battery gets to 14.4v, then they switch to constant voltage mode and drop the voltage to like 13.8v for a timed period - say an hour - to allow the battery to "absorb" (absorb is stage two). Then they go to the "float" stage (stage three) - still in constant voltage mode - and just hold the battery (float it) at a constant voltage (normally around 13.2v). Since holding the battery at 13.2v can be done forever without hurting it, you can leave the charger connected and let it "float" the battery 24/7.

2-stage chargers just do the bulk stage and then drop immediately to float. In float mode, they are constant voltage chargers and can be left hooked up indefinitely.


Most cheez-whiz constant voltage chargers like yours and mine float the battery at a fairly low voltage - 12.6 in our case - so they can be left hooked up without hurting the battery. They *shouldn't* be, because at some point the battery needs to get pumped up to 14.4v and our cheezy little chargers just can't do that. For a vehicle, it's okay since the alternator can pump up the battery to a proper voltage once in a while.

If you were charging say a solar battery that never gets used in a vehicle - then our cheezy chargers at 12.6v would be just too wimpy and never getting the battery truly full would allow sulfation to build up and lead to early battery failure. But even so, floating at 12.6v with a cheeze-whiz charger would still be better than sitting around slowly self-draining and sulfating.

 

[alexrex20]

Hate to say it, but there is a very high chance that it may be junk.

If it's been rode hard and put away wet - AND/OR left sitting around uncharged - AND/OR drained down to 5v - then it DOES have hardened sulfate crystals. The question is how much.

(GOOD LORD MAN! FIVE VOLTS? THAT'S HORRIBLE!...seriously...I'm not joking...12v batteries are considered completely dead at 10v...5v would be like...practically a negative number. I'm surprised that battery hasn't imploded and become a Black Hole.)

Well, that's not exactly what I wanted to hear, but I'm glad you told me before I invested any time/money into installing this battery. The battery stabilized at 12.36V overnight, so it's back on the charger again.

The manufacturer lists a Capacity Test in the manual. They hook up a constant 20A draw and measure how quickly it drops to 10V. Is that something I can replicate with say, a 200W lamp? That would be about 17A draw at 12V.

Or should I just cut my losses and junk the battery? I guess if "they" were so quick to discard them, then they probably know something I don't...



Thanks for the excellent explanation of the different types of chargers

 

[DeMented Toys]

When using the simple continuous open solenoid with a manual switch, how do you keep from overcharging the auxiliary battery bank? Is overcharging possible from the alternator?