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Old 09-01-2015, 12:21 AM   #1
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battery question

We have two 6V 230 amp hour golf cart batteries that we had wired to an isolator. After we discharged them the first time to around 50% we were never able to get them back to full charge. We kept taking readings and at resting stage they would put off about 12.43 on average. After a long drive they were reading 12.5 in a resting state. When we got them they read 12.7 ish

My question is this: because the two 6V's are deep cycles is charging them off our big old bus alternator damaging them. I read somewhere that deep cycles should really be slow charged for long periods to get them back to full charge.

Also, what is an acceptable level of discharge on the deep cycles? The online consensus seems to be not below 12.3

Does anyone have any experience with problems charging deep cycles off the alternator?
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Old 02-07-2016, 09:09 AM   #2
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Depends on your isolator type; for semiconductor type, the SCR/Diode will consume minimum of 0.2 volts drop and varies with the charging current across the blocking device, and thus your battery will never see full charging voltage coming from your alternator. That's how diode work unless manufacturer use more expensive Schottky Diode. This diodes costs more to fabricate than blocking types or SCRs, typically used in electronics device where overall efficiency is critical. Relay type has minimal drop, but these devices typically have short life span.

Your battery is charging as long as your alternator is engaged, although it will never be fully charged. As far as your alternator, it doesn't matter how many batteries are connected or the type of battery used. Alternator will produce constant current and voltage regardless of your load. Your batteries, especially the high capacity one, will take a lot longer to charge. This is another reason why it become less desirable to charge your expensive battery with vehicle's alternator as explained in following paragraph.

All battery charger, regardless of types, will be handled in three distinct charging phase. You want your charger to provide constant current until battery reaches full voltage, then switch to constant voltage mode, give juices as chemical in the batteries stabilizes. Lastly, once battery reaches optimal charging voltage and stabilizes at set point, you don't want to overcharge your batteries, hence your charger needs to operates in tickle charge mode, similar to constant voltage mode. This charging algorithm is used all batteries types; lead type, glass gel or lithium all obeys to one universal algorithm.

Do all battery charger obeys the same rule? NO. Some cheaply made Chinese labeled don't, and it's often difficult to get the correct information from customer service dept, and therefore, insist you want to talk to a real engineer who understand electronics. Everyone wishes to call themselves as an engineer, but not too many people knows their material. It's a reality.

If you have such large capacity battery, it would be prudent to have a dedicated charger for auxiliary battery only, because typically alternator is not designed to produce current needed beyond vehicle's primary battery. You won't need to be concerning with your wiring because your isolator will do what it suppose to do, that is, blocking charging current to vehicle's primary battery. Otherwise, you're going to subject to overcharging vehicle's battery, that will just about obsolete the isolator, but you won't have to worry because isolator will take of it for you.

Some people argue having an isolator charging two separate battery of different volatge is not the right way of charging, creates all kinds of complexity for your vehicles' electronics. Technically, this argument makes sense when you think about it, but not all electronics are made same, some built with higher tolerance to abnormalities.

As far as how deeply you want to discharge your battery, it's your choice. You trade off how long you want your batteries to last vs. how often you want to charge your battery. East Penn Manufacturing, one of the largest battery manufacturer in the US has an excellent resources, technical documents that will excite your brain, showing all kinds of graphs, and their customer service department has one of the most knowledgeable and technically capable staffs I've death with. It's very rare a company will hire technically capable staff for Customer Service group, but this company is different. If you cannot find an answer you're seeking, I suggest you call them, spend hour or more discussing your situation, you'll be educated, in a level you think you're an expertise in complex chemical engineering of batteries.
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Old 02-07-2016, 11:28 AM   #3
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The diode isolaters are known to undercharge batteries for the reason Kwang posted.

Charging starting and deep cycle batteries from the alternator can be a problem because starting batts. like to be charged hard and fast while deep cycles want a slow extended charge.

Another issue is that the alt. can't choose which bank to charge and will attempt to top off the bank with the lowest charge. This can cause the other bank to be over charged and boil off electrolyte.

Charging a large battery bank or a heavy, extended drain while driving can cause an alternator to go "full field" (run at full output) for extended intervals and burn out the alternator. Your small house bank wouldn't cause this.

Alternators monitor the battery banks and adjust voltage and amperage according to the state of battery charge. My digital battery monitor shows 14.9 volts just after starting which tails off to around 13.5 after an hour or so of driving.

We're weekend warriors so I charge my starting bank with the alternator and charge the house bank with our Progressive Industries 3 stage converter/charger when we can plug in. We don't use much power on our trips so this works for us.

So, what to do?

You could keep your diode isolater for adequate charging on trips then use a good 3 stage charger to fully charge when shore power is available. Given that you already have the isolater, this is the way I would go.

Another option is a manual battery switch that lets you choose what gets charged (and when) or a small generator to charge when away from shore power. Both have downsides; With the manual switch You have to monitor the the battery banks and you could potentially leave the switch in the wrong position and drain both banks on a camp out. The generator option requires you to buy a generator and haul the generator and feed the generator and annoy those around you with the generator. lol

A 50% discharge is about all you want to see for good deep cycle battery life. A given deep cycle batt. may have 150 deep cycles (full charge to full discharge) while the same battery only 50% discharged may have 3 - 4 times that many cycles. Basically, the lower the discharge between charges the longer the life. A full charge is ~12.7v and a 50% discharge will read ~12.1v.
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alternator, batteries, charging, golf cart batteries, isolator

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