Current Draw & Charger Question

[Ward H]

I tried to figure my current draw, charge time and amperage to figure if I am running on borrowed time with my battery without a good charge from a source other than the OB.

This winter I plan to add a second battery as a reserve/emergency battery but as I get more sailing time in, I wonder how much I am pushing my current setup. I've considered an emergency jump box but would rather put the money into a charger and second battery. I just don't want to take the time and money to do that now.

Here are the facts.
We only day sail our O'day 25 and our only current draw is:
GPS - 1 Amp
Auto Tiller - 1.5 Amp (Used maybe 50% of sailing hours)
Depth Finder - 1 Amp (1 Amp fuse so guessing less than one amp draw)
3 Hrs sailing = 8 Amp Hr draw

OB Power Tilt & Start - 100 Amps for 15 seconds at most twice each sail. I figured 1 Amp Hr max current draw for the OB.
So a total of 9Amp Hr draw for a 3 Hr sail.

OB charges at max of 6 Amps. Since it mostly idles or runs at 50% throttle I am guessing it might put back in 2 Amp Hrs for the avg .75 hrs it runs per 3 hr sail.

That leaves me with a deficit of 7.5 Amp Hrs per day sail.

Battery is a 4 yr old group 27 but I am just guessing it is around 75 Amp Hr.

So, based on this my battery should be dead after about 10 day sails. Even if I shortened the avg day sail to 2 hours it looks like I might get 11 day sails out of the battery.

I just finished by 27th day sail for the season. The battery was last charged and topped off when the boat was launched in May.

So apparently I am using much less amperage draw than I am figuring or the OB is much better at charging than I expect.

Did I go wrong with my numbers somewhere?

Should I use my 30 yr old auto charger to carefully give the battery a quick charge to get me through the rest of the season?
I have about another dozen sails planned before my season ends.

 

[Sailm8]

Have you considered a small (like 3amp) solar charger to run through out the week?

 

[Benny17441]

It's probably both, you are drawing less power than projected and receiving more charge than anticipated plus a healthy group 27 lead /acid has around 100 ah not 70. It is not good for the useful life of the battery to discharge beyond 50% of its rated capacity and to allow it to remain partially discharged for long periods of time. Get a digital volt meter and take a reading across the terminals after allowing the battery to rest for 2 hours (no loads, no charge). The voltage will approximately indicate the percentage of charge left on the battery.

12.6+ V 100% 100A
12.32 70% 70A
12.06 50% 50A
11.75 30% 30A (not recommended)

Definitely recharge your battery now for those outings you have planned before the end of season. Even if your battery still has enough charge to carry you through it does no good to continue to leave it partially discharged. Your electronics use very little but the draw on your auto pilot is dependent on weather conditions (how hard does it have to work). While you are at it check on what type of battery you are using, whether a "starting battery" or a "deep cycle battery". The first is usually rated in cold cranking amps and the second in its capacity of Amp Hours. A "deep cycle battery" has thicker plates and provides a large number of discharge/charge cycles while a starting battery is designed to produce a burst of power for a short period of time and do not take kindly to deep discharges.

 

[Ward H]

SailM8,
Solar is under consideration but I have enough stuff hanging off the back now so it is not top on the list.
Benny,
Memory failed me on this one. It is 2009 Lee 24M7 battery, which appears to be a starting battery. I've read how a deep cycle would be better so I will put a new deep cycle battery on my spring prep list and consider going up to a group 27. I saw a group chart which indicated a group 24 is around 70 AH and the 27 is around 100 AH. That should work well.

For now I found where Maine Sail recommended a Guest 2606A portable 6 Amp charger so I got one coming for this weekend. I'll use it for topping off whatever battery I have a few times during the season.

Thanks for the voltage/% charge list. I'll start checking that once in a while to get a feel of where % charge.