Matching charge rate to battery capacity

I have been reading the current posts on chargers. I admit after about half a dozen I am completely out of my depth. I do have a query, but feel that a hijack there would not be well received. So a new thread.

The thing is, I keep coming across posts where people have put huge alternators etc into their boats with large amperage capacity to recharge their battery bank systems.

I have the old & trusted lead acid batteries and have always been told match your charge rate to battery capacity carefully, the figure I grew up with was between 10-15% of the battery capacity. So does this still hold true.

I know other more sophisticated batteries systems can take a greater charge rate, but this query is mainly about the lead acid types.

Andy: I believe 10-15% is still good & true for conventional lead-acid ('flooded') batteries...which for House application still represent the best $$/Ah value & are fine so long as their location allows for easy access to water-fill caps. My shorepower charger is set at 93A max...10% of my House bank capacity. Note however that with alternators charging the battery, you ought to have an external 3-stage regulator to manage the charging current. These range from Basic/Fixed to fully programmable.

What are you thinking of upgrading/changing?

Aquabelle said:

Andy:
What are you thinking of upgrading/changing?

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No not really, the house bank was replaced about a year ago, and I'm just getting used to the new Victron(2,000watt) that charges them.I wanted to make sure the charger was set at the right charging amperage.

A battery monitor will indicate the charge rate in amps from both your battery charger and the alternators. Call the manufacturer of your batteries to determine the recommended charge rate. You can then determine if adjustments are necessary.

The best way to determine re-charging and discharge rate is Peukerts equation for charging and the general rule of thumb do not discharge below 50% of the deep cycle battery capacity.
What is your wet cell batteries AH capacity?
Perhaps I can run the numbers for you?
Bill

No worries with the victron. Having a modern charger that is oversized will cause no harm, other than possibly spending more money than you needed to. All modern chargers regulate based on the battery voltage. If the battery is getting too much current, it's voltage rises. That will be detected by the charger and it will back off until the voltage gets back to where to should be. It's like the governor on your engine that varies the throttle to maintain RPM regardless of load.

twistedtree said:

No worries with the victron. Having a modern charger that is oversized will cause no harm, other than possibly spending more money than you needed to. All modern chargers regulate based on the battery voltage. If the battery is getting too much current, it's voltage rises. That will be detected by the charger and it will back off until the voltage gets back to where to should be. It's like the governor on your engine that varies the throttle to maintain RPM regardless of load.

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That's partially true but if you don't tell many of the modern inverter/chargers your battery AH's it really has no way of knowing other then measuring the static voltage of the battery bank.
Bill

For ordinary FLA batts, my understanding was that a charging ability (in amps) of 25% of the bank's AH capacity would be ideal, as 25% is the max acceptance ability of FLA batts.

RCook said:

For ordinary FLA batts, my understanding was that a charging ability (in amps) of 25% of the bank's AH capacity would be ideal, as 25% is the max acceptance ability of FLA batts.

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This statement is only partially accurate, sure if you only discharge to 25% your batteries will live longer but 25% is not near the maximum discharge for FLA deep cycle batteries.
Bill

Not what I meant - of course you wouldn't want to discharge to below 50% of capacity.

The charging RATE (amps) could be 25% of the capacity of the battery bank as measured in AH. For example, my house bank is 200 AH, and my Freedom 10 (a three-stage smart charger) charges at 50 amps max. Just right.

Andy, another thing to keep in mind is the on engine alternator does more than charge the batteries. Dependent upon your wiring setup, lots of amps go directly to running onboard stuff whether instruments or appliances. I've seen it often where alternators will charge the house bank very slowly as the other draws are quite heavy. For this reason big alternators come in handy.

I've yet to hear "my alternator puts out too many amps!"

Wait I challenge your just right. What do you mean are you saying a 50 amp charger is just right? For what a 200AH battery bank? A 10 amp charger might be just right for some people it will just take longer to charge a battery or battery bank assuming there are no active devices on the 12 or 24VDC buss. A 100 amp smart charger would re-charge faster then a 50 amp charger. The hardest part of the charge cycle isn't the bulk charge it's the final charge the last 5 to 10% that takes time and is the point where any sized smart charger is outputting reduced current.

Billylll said:

A 100 amp smart charger would re-charge faster then a 50 amp charger.

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A flooded lead acid battery won't safely accept more than about a 25% charging rate. So in my case, the 50 amp smart charger is about as good as it gets.

A quote from the Heart/Freedom owner manual:

Generally, a wet cell battery bank should
not be charged up to the gassing point at a
rate which exceeds 25% of its capacity.
Example, a 12 Volt battery bank of 520
Amp-hours should not be charged at over 130
Amps.

Smart battery chargers and alternator V regulators also monitor the TEMPERATURE as the set is charged.

25% to start might be fine ,but its better to not worry and let it just happen automatically.

Of course if the charger is tiny , 10%, no temperature monitoring should be required .

There has been considerable debate over low current charging sources and if they need to be smart or measure the ambient battery temperature. I'm with the camp that says you should always use a multistage charger and a temperature monitor. Unless your vessel has static loads, ie; a DC frig, an automatic water maker even a large battery bank can be destroyed with a small solar or a constant voltage (ferro) type battery charger.
Bill

Slight thread hijack.

Why is 50% considered,if indeed it is, the tipping point in discharging deep cycle lead acid batteries.

I have a modest 400amp deep cycle set up,which also starts the engines, with inverter and genset.

I get a bit frustrated that I really only have a usable 200amp supply if I don't want to damage the batteries.

Andy G said:

Slight thread hijack.

Why is 50% considered,if indeed it is, the tipping point in discharging deep cycle lead acid batteries.

I have a modest 400amp deep cycle set up,which also starts the engines, with inverter and genset.

I get a bit frustrated that I really only have a usable 200amp supply if I don't want to damage the batteries.

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You can discharge to 60% but it will impact the life cycle and span of the batteries. I know people that take FLA's down to 70%. I wouldn't suggest it other than occasionally. If you want to reduce the life of the battery bank to a few years discharge them to whatever point your comfortable at.
You may find the need to top off the fluids more often and even equalize them but a heavy discharge cycle repeated generally ='s short battery life.
Bill

Its a trade off between DOD (depth of discharge ) and how many times it can be done.

Google Trojan and see their graphs.

Remember ALL are created with the concept the batt will be discharged then immediatly recharged to 100%.

AS 100% full is seldom done on a cruiser , for longest life a de sulphation unit will help.

If you look at those life cycle vs depth of discharge curves, what they are really telling you is the the battery can deliver a fixed number of ah over it's life. There is an exception for very deep discharge to around 20% or lower, but let's ignore that for now.

You get to decide whether to consume those ah in small chunks over many cycles, or in large chunks over fewer cycles. The choice is yours. The whole 50% depth of discharge for "good" battery life is really pretty arbitrary. The cost is about the same if you use fewer batteries, discharge them more deeply, and replace them more often, compared to more batteries with less DOD and longer life. There actually is a strong argument for small banks/deep DOD/frequent replacement, namely there is less time and opportunity to ruin the batteries via other mechanisms like chronic under charging which can be a real problem on boats.

<There actually is a strong argument for small banks/deep DOD/frequent replacement, namely there is less time and opportunity to ruin the batteries via other mechanisms like chronic under charging which can be a real problem on boats.<

Would not the smaller batts be further down the discharge cycle with the first amp drawn? Adding to the time the batt set is at lower voltages , aiding sulphation?

Many cruisers will go days between charges while on the hook.

Those without electric fridges/freezers can go a week or more.

As long as you bring them back to full charge every few days or once a week, it shouldn't make any difference. Sulfation is more about how long they are left discharged rather than how deeply discharged.