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Old 17-09-2018, 05:00   #1
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Will a larger battery bank maintain voltage level better than a single battery?

Earlier this year, my battery bank died due to old age. As my current cruising schedule is limited to a night out here and there, I decided not to replace the full 12v house bank bank until I set off cruising full time. In the interim, I fitted a single 200ah 8d deep cycle battery which, given my large solar array is sufficient.

This week I finished the install of my electric winch. The trouble is, as I discovered on its first trial yesterday, when the winch is under load it draws sufficient amps to make my Raymarine chart plotter power down. (They are both powered from the house bank). Although I haven't measured it yet, I'm guessing this is due to a voltage drop in the system. Once I fit my new bank, will the increased reserve likely prevent this from happening?

I'd appreciate any thoughts.
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Old 17-09-2018, 05:05   #2
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Re: Will a larger battery bank maintain voltage level better than a single battery?

You at probably drawing more than the battery can deliver. It’s also possible your wires to the high draw are relatively too long and too small. Both contribute to your problem.
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Old 17-09-2018, 05:22   #3
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Re: Will a larger battery bank maintain voltage level better than a single battery?

Possibly your battery is not up to the task... but... I'd expect an 8D battery to do the deed.

Before running out to get a new one, I'd check the connections. Just one high resistance connection between the battery and where the chart plotter branches off from the main buss would be enough to drop voltage a lot during high amp draw.

When you are using your windlass are you running your engine to keep the alternator running? If you run the engine at 1500RPM or higher the alternator will supply much of the power the windlass needs.
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Old 17-09-2018, 05:40   #4
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Re: Will a larger battery bank maintain voltage level better than a single battery?

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Possibly your battery is not up to the task... but... I'd expect an 8D battery to do the deed.

Before running out to get a new one, I'd check the connections. Just one high resistance connection between the battery and where the chart plotter branches off from the main buss would be enough to drop voltage a lot during high amp draw.

When you are using your windlass are you running your engine to keep the alternator running? If you run the engine at 1500RPM or higher the alternator will supply much of the power the windlass needs.
Good advice on terminals and the engine. We never operate the windlass without the engine. I want to be sure we have control before drifting randomly as well as making sure there is enough powere to start.
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Old 17-09-2018, 05:51   #5
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Re: Will a larger battery bank maintain voltage level better than a single battery?

Yes, a bigger high AH capacity bank will carry higher-current loads with less voltage sag.

FLA usually best value and longevity, but higher resistance / lower CAR implies bigger capacity required

than quality AGM.

Odyssey "dual use" TPPL tech is actually brilliant for deep cycling unlike that label with cheaper batts that are just beefed up starters.

And puts out a lot of amps on demand without having to go as big a bank - but obviously longer #minutes output requires proper sizing so you don't discharge too deep, stay above 50%.

Example G31 Odyssey PC-2150.

Lifeline / Northstar also good, same idea and tech.

But do **not** get away from true deep cycle just to get "cranking power", cheaper "dual use" won't last with the heavy usage.
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Old 17-09-2018, 05:53   #6
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Re: Will a larger battery bank maintain voltage level better than a single battery?

Thanks guys. Itís a sailing winch so I rarely have the engine running at the same time. Iím pretty sure that the wiring is sound. Itís all new and properly sized but you make a good point. I will check it first for anything loose.
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Old 17-09-2018, 05:53   #7
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Re: Will a larger battery bank maintain voltage level better than a single battery?

And best for high current loads, minimum voltage drop is LFP, can last very long but very pricey up front, 5-7x the cost of even the best lead.
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Old 17-09-2018, 05:57   #8
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Re: Will a larger battery bank maintain voltage level better than a single battery?

What 8D did you have?

Ideal would be concurrent ICE input supporting the load even if most of it carried from the bank, don't need as much bigger AH capacity.

Since this is House / nav etc as well, the other big bonus is shallower cycling, longer lifetime.
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Old 17-09-2018, 08:45   #9
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Re: Will a larger battery bank maintain voltage level better than a single battery?

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Originally Posted by Privilege View Post
Thanks guys. Itís a sailing winch so I rarely have the engine running at the same time. Iím pretty sure that the wiring is sound. Itís all new and properly sized but you make a good point. I will check it first for anything loose.
Just for checking, what size cable did you use and how long is the run from the battery to the windlass?
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Old 17-09-2018, 08:49   #10
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Re: Will a larger battery bank maintain voltage level better than a single battery?

The bad news is the 8D battery that you bought is most likely not a deep cycle in spite of how it was labeled.
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Old 17-09-2018, 08:59   #11
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Re: Will a larger battery bank maintain voltage level better than a single battery?

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Just for checking, what size cable did you use and how long is the run from the battery to the windlass?
I’ve got 00 cable running to a bus bar, a run of around 20’ one way. The winch is then supplied by 0 cable for the next 6’.

The winch is a 50 and draws a maximum of about 75amps.
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Old 17-09-2018, 10:18   #12
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Re: Will a larger battery bank maintain voltage level better than a single battery?

Yes, because of the Peukert effect: https://en.wikipedia.org/wiki/Peukert%27s_law

A 100 AH lead-acid battery will deliver 5 amps for about 20 hours (100 AH). The same battery will deliver 1 amp for about 110 hours. So, you get a capacity "boost" of 10% at the lower draw rate. If you had a 500 AH battery with a 5 amp load, you'd get about 550 AH capacity. For a given amperage load: the bigger the battery, the more is the apparent advantage due to the Peukert effect.

This is why I don't have a "main/aux" switch between my two battery banks. Instead, I have a battery combiner using shottky diodes: the Peuket effect gives me more useable capacity by spreading the load over both battery banks, while preventing a discharge from the lesser-discharged bank into the higher-discharged bank. Heavy momentary loads are drawn from only one battery bank, allowing the lesser-loaded bank to maintain a terminal voltage useable for the electronics. I use a second set of shottky diodes in the charge path, so charge current is routed to the greater-discharged bank, until both banks are balanced.

I also use a battery booster buck regulator to maintain a minimum voltage of 13.0 volts into my autopilot, chart plotter, and SSB transceiver; regardless of my battery voltage (down to 10.5 volts). All of my autopilot problems went away once I implemented that, and my SSB maintains 100 watts output regardless of battery state. But the dual bank isolation scheme I described above, with high momentary loads drawn only from one bank, will prevent your electronics from malfunctioning during momentarily high loads, while giving you the advantage of both banks for ordinary loads.

My "heavy load" bank consists of two Odyssey batteries, and the "light load" bank has two Lifeline batteries. The schottky diode combiner also allows me to use the higher 14.7 volt absorption voltage the Odyssey batteries want, while using the 14.4 volt absorption voltage the Lifelines need (both voltages are temperature compensated).
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Old 17-09-2018, 10:19   #13
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Re: Will a larger battery bank maintain voltage level better than a single battery?

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Originally Posted by Privilege View Post
I’ve got 00 cable running to a bus bar, a run of around 20’ one way. The winch is then supplied by 0 cable for the next 6’.

The winch is a 50 and draws a maximum of about 75amps.
That is a long run, 52'. It sounds like the current drain is just dropping the voltage enough that your electronics do not like it. If you're considering another battery can you add one closer and dedicated to the winch.
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Old 17-09-2018, 10:29   #14
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Re: Will a larger battery bank maintain voltage level better than a single battery?

It's better to use a dedicated starting battery to power hi amp loads like starter or windlass to avoid voltage drops that affect electronics. Also, with the engine running the alternator should supply most of the power to the windlass.
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Old 17-09-2018, 10:58   #15
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Re: Will a larger battery bank maintain voltage level better than a single battery?

using the Blue Seas ampacity chart
http://assets.bluesea.com/files/reso...on_chartlg.jpg
and a max load of 75 amps over a distance of 60 ft, the wire size should be 3/0 for a 3% voltage drop.

You said you had a combination of 2/0 and 0 which would put your system into a voltage drop of more than 3% and less than 10%, if all of your connections are ideal.
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