Charging Calculations

[Ford Prefect]

Hi guys. Trying to figure a few things out and not sure if my math is skewed. Not an electrical guy t all, so help me if I'm off somewhere.

I have two Energizer 27DC that have combined a reserve capacity of 360. Dividing that by 60 (minutes) and multiplying it at and amperage of 25, I'm estimating 150 amps available.

When I'm off shore power, I'm estimating my daily usage of 77 amps, taking my fridge offline and using it as an icebox.

My alternator has an output 35a. Dividing the 35 into the 77 amps used, I'm calculating that I'll need to run my engine approximately 2.5 hours per day to keep my batteries topped off

Is my math fuzzy? I'm going down to Biscayne Key for a couple days and plan to stay at anchor. Just trying to make sure I don't get myself stuck.


Oh ya, last question; do I use more amperage, less amperage, or the same if I'm using appliances, laptop, etc with an AC inverter plugged into a DC outlet over using the wall sockets?

Thanks for your input, all!

[accomplice]

I'm surprised your group 27 batteries have 180Ah each. Usually deep cycle group 27 have about 100Ah capacity (at a 20hr rate). Assuming your batteries are typical, you would have 200Ah total capacity with two batteries. However, if you don't want to significantly shorten the life of the batteries, you should draw them down no lower than 50%, or 100Ah from full charge.

Charging batteries is not a linear process. The regulator that is built-in to your alternator may not be very sophisticated and is unlikely to apply a topping or float charge. If it only applies a constant-current charge, you may only charge to 70% of battery capacity. If this is the case, you may be charging to the 70% point and discharging to the 50% point -- only using 20% of capacity, or 40Ah.

Based on this calculation, and the assumption that your needs consume 77A-h/day, you would need to run your engine for approximately 1-1/4hrs two times a day.

Do you have any information on your alternator's regulator?

Regarding use of an inverter -- you obviously would use more electricity with it and appliances attached than without them attached/on. Perhaps, though, I misunderstood your question.

[a64pilot]

If you have 120V available, you have either shore power or an inverter. If yu have shore power it would be better to use the 120V directly as opposed to pulling 12V and using an inverter, although it's not enough different to be very relevant as if you have shorepower, you have a battery charger too.
Is that what your asking?

You know of course your the perfect candidate for solar with amp usage that low.

[Ford Prefect]

Thanks Acc. The batteries are rated at 180 RC, not Ah. To figure the Ah, I'm using the above calculation of dividing the RC into 60 then multiplying by 25. Something I read a bit ago, which seems more conservative than the 100Ah.

Not sure about the alternator's strength, but 70% sounds a safer bet. That brings it to 3.2 hours a day of charging, or a little over 1.6 hrs twice a day, as you suggested.

Yes, that answers my question, a64, thanks.

Solar is the next investment

[djmarchand]

The 35 amp rating of your alternator almost certainly implies that it is an automotive type that supplies a constant voltage, not current as the previous poster indicated. You will be lucky to get 10-20 amps out of this alternator even starting with a 50% discharged battery. The alternator puts out a constant 13.8 volts for example. When the battery is 50% discharged, it's voltage is 12.0 so you have 13.8-12.0 = 1.8 volts driving amps into it and you may get 20 amps. Within a few minutes the battery gets up to 12.5 or so and you only have 1.3 volt so the amperage drops to 15 and then after a while 10 and then a few amps after a while and you are still only 80% charged. It takes many, many hours to get that last 20% with that kind of alternator.

The only way to avoid this is with a high output alternator and an external 3 step regulator.

OTOH, I doubt seriously that you will need 77 amphours per day with no refrigeration. More like 30, unless you somehow surf the net (from where on the hook?) on your laptop all day long.

In any case running your propulsion engine for 2,3,4 hours each day to recharge your batteries isn't good for it. Unless this is a very occasional thing, I would invest in solar and more battery capacity.

David

[Autumns Wind]

Adding to his Question... what's the sure test for checking to see if the alternator is even charging the batteries?

[accomplice]

AutumnsWind, the surest test is an ammeter inline.

The simplest test is whether the (battery) voltage goes up when the engine is running. It doesn't tell you charging rate, but it is a binary indicator of operation.

[a64pilot]

engine running well above idle, voltage should be between 13.8 and 14.2, this is for a stock internally regulated alternator

[Stu Jackson]

Battery Acceptance by Stu Battery Acceptance

[sailinglegend]

Originally Posted by Ford Prefect

....Is my math fuzzy?...

YES - and you need to understand the difference between AMPS and Ah or amps-hours. You use 77 Ahs per day, that's 7.7/24 = 3.2 amps for 24 hours, or 77 amps for 1 hour!!! 77 Ah is not very high, the average cruising boat can easily use 150Ah/day

The RC you quote of 180 minutes is the time you have left in your battery to get you back to the garage if the alternator stops charging and you are drawing 25 amps. RC is usually quoted for CAR BATTERIES and not true deep-cycle boat batteries.

The Ah capacity of your battery is probably 115 Ah - that means with two batteries you have only 115Ah to use before the State of Charge (SoC) goes down to 50%. But if you charge with your engine you will only ever get to about 85% SoC, unless you run it all night, so you only have 50%-85% = 35% of the 230Ah available which is about 80Ah.

So to replace 80Ah with your 35 amps alternator will probably take about 4 hours every day because the output falls as it gets hot. If you only charge to 85% the batteries will die quickly because they are not getting fully charged regularly.

So get a proper alternator with external regulator and a good shorepower charger to get back to 100% as often as possible. Lots and lots of solar will also help.

[Snore]

Originally Posted by sailinglegend

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So get a proper alternator with external regulator and a good shorepower charger to get back to 100% as often as possible. Lots and lots of solar will also help.

Not being persnickety - but what would you deem a "proper" alternator? 50-100?

[hellosailor]

Ford-
Usually when a battery spec includes "reserve capacity" it is an SLI (starting, lights, ignition) battery not a deep cycle battery. Deep cycle batteries pretty much always have an amp-hour rating.

So forgive me for asking the obvious, but DO you have deep cycle batteries? Regular auto batteries? Or some "dual purpose" kind?

A conventional SLI battery can be killed in just a few very deep cycles, an expensive surprise.

[Ford Prefect]

Hello, nope. All I've got in board are those two. From what you and legend are saying, it sounds like I have maybe a day at best of being away from shore power.

[hellosailor]

Well, interstate shows their G27 deep cycle battery as having a 180 minute capacity under a 25A load. Which would be 75AH, which is about what I'd expect from a Group27. With two of those in parallel then, you'd have 150AH and for occasional use (not regular use) I'd personally be comfortable pulling maybe 100-110AH out of them before recharging. Assuming the voltage was holding and the starter motor wasn't protesting. With typical light loads (stereo, lights, no Jacuzzi heater though) you still probably would want to keep an eye on the battery voltage and not let it go below 12.0 volts.

Not so much because of questions of battery abuse (unless they are SLI batteries) but also because somewhere between 12.0 and 11.6, your starter may decide it has been offended.

You'd have to work up an energy budget based on what you've got and what you use, to know if that's one day or more.

[sailinglegend]

Originally Posted by Snore

Not being persnickety - but what would you deem a "proper" alternator? 50-100?

Get one that is 25% of your battery capacity - so if yours are 115Ah each - and I couldn't find their actual capacity - then you need a 60 amp alternator. More importantly you need a "marine" hot rated alternator that will actually give you 60 amps when hot. Car alternators are not designed to charge deep-cycle batteries, but they are very cheap. You also need a multi-stage external regulator to charge the batteries properly. Search this forum to find out what this really means!

I'm afraid these items are expensive, you just have to decide what you want to spend your money on.