# Which diode for boat use

1. May 9, 2014

### anorlunda

I want to connect a lead-acid starting battery to a bank of deep discharge batteries on my boat via a diode. The purpose will be to let the start battery charge from the same source, but to not back feed.

First, I'm unsure how to rate the diode. 12v is maximum voltage, that's clear. But should I rate the diode equal to the max amps of the charging source (75 amps). I fear surges of higher currents when connecting batteries of unequal charge state. In actual use, it will probably conduct 20 amps for 3 seconds after each start of my diesel engine. It only takes about 400 joules to start the engine.

Second, the diodes I find online seem to all have tiny pins made to solder into a printed circuit board, even though some claim to be rated 75A. It seems ludicrous to connect such a tiny pin in series with 6AWG cable. Also, I was also taught long ago to never use solder in power circuits because of safety reasons.

What is the right diode for me?

2. May 9, 2014

### AlephZero

You don't need the starter motor current to flow through the diode. That is a very bad idea. The diode will have a voltage drop across it and reduce the voltage getting to the motor, and if the starter is jammed one day, it will draw hundreds of amps from a fully charged battery, not your estimate of 20.

Look at a wiring diagram for a car electrical system. The starter motor and switch are wired directly across the battery, separate from everything else.

I'm not sure what "diodes you found online", but this is what a typical 75 amp diode looks like: http://store.mwands.com/solar-components/75-amp-600-volt-blocking-diode/ [Broken]

Last edited by a moderator: May 6, 2017
3. May 9, 2014

### anorlunda

Thanks Aleph Zero.

I was trying to avoid drawing a picture. My goal is to run only the starter from the starter battery, all other loads served from the main battery bank, yet to have a common source for recharging the batteries. When charging the voltage rises to 14.2 volts, and the starting battery can follow along at 13.9 volts or so. That's plenty for full charge.

The link you provided to a candidate diode looks just right. Thanks again.

4. May 9, 2014

### sophiecentaur

To isolate each battery from the other, you need a diode in series with both batteries. However, because of the half volt drop across the diodes, the normal alternator regulator will not provide the right voltage for the batteries to be completely charged. You need to remotely sense the battery volts to make the regulator work correctly.
Alternatively, a Voltage Sensitive relay can be used. The alternator charges the Start battery which, when the Start battery is sufficiently charged, will connect the domestic bank. This ensures rapid charging of the 'important' battery and reliable starting of the engine. A VSR is not too expensive. (Well, not too expensive if you reckon you can afford to run a boat in the first place. I feel your pain)

5. May 9, 2014

### jim hardy

Aleph is right.

When cranking, a good battery can drop to nine or ten volts.
In response your diode will allow the main battery bank to feed current to the starter during cranking.
One day , when a cranking battery terminal corrodes and your main bank tries to start the engine by itself through the diode it will destroy a 75 amp diode.

Here's a 400 amp diode that stands a better chance of survival
http://www.vishay.com/docs/93510/vs-400urseries.pdf
around seventy bucks at Digikey.

Sophie's relay would be a better choice from standpoint of charging the start battery to correct voltage.
Myself i'd control the relay from the engine's oil pressure switch so the batteries are paralleled only while engine is running, and add a manual pushbutton or heavy duty battery connector switch to allow emergency cranking from main battery. (Make sure it's sized for starting current.)

old jim

Last edited: May 9, 2014
6. May 10, 2014

### sophiecentaur

And you'd probably make it work, too!! . But the problem with that 'open loop' form of control is that it ignores the state of the engine battery.
The domestic supply should be very much 'second place' to the start supply. When you've had a day living on board without the engine, you want to be sure that the start battery is tip top. If you just connect the two in parallel, irrespective of their states, the domestic one could be hogging all the charge current and the start battery could not be getting into a good state after a short engine run (as short as possible for a sailor, who turns that stinky engine off asap, when there's any wind at all). `A sagging domestic battery will make itself known in a harmless way and you have time to do something about it by running the engine.
Either way, the 'diode block' (always TWO diodes - one each) that people used to buy from the chandler's is not the way forward on a modern boat with a domestic load that is several times bigger than it would have been, twenty years ago (Radar, TV, Fridge etc). The most advanced systems are very clever but cost ridiculous money - with the word 'Marine' in their names.

7. May 10, 2014

### Baluncore

The main problem with a PN junction diode in battery circuits is junction forward voltage drop. These days we use power mosfets instead of PN diodes. The mosfet is installed so the intrinsic diode is performing the job of the diode. However, a voltage comparator between the two battery systems controls the gate, so the mosfet is either off with it's intrinsic diode reverse biassed, or the mosfet is turned on which shorts the forward biassed intrinsic diode. This reduces power consumption when conducting by significantly reducing the switch voltage drop.

When operating the starter motor, the system should separate the battery banks. The intrinsic diode cannot do that but by fitting another series mosfet in the same way, but reversed, you can control the connection like a simple relay or switch. The two mosfet gates are controlled by the digital state of the starter control and the voltage comparator. You can mix any logic in there you need to suit your application and optimise reliability.

Consider a mosfet with very low Rds on, so it will have a low voltage drop. I use IRF1405N up to 40 amp. I avoid it's specified 169 amp maximum current, while driving the gate with at least 15V to get the very low Rds on. IRF1405N is automotive rated to 55V.

You might consider making a controller for a commercial “DC solid state relay”, which these days will contain two mosfets as above. That will probably be a more expensive solution.

When building parallel battery systems, it is important that the battery technologies are matched or there will be a difference in charge voltage and the system may not behave or charge properly. It can be very difficult to diagnose when someone replaces a battery with a slightly different design. Above all, be aware that there can be differences in charge voltage between a high CCA battery for starting and a deep cycle battery.

8. May 10, 2014

### sophiecentaur

Standard practice amongst the 'wealthy' Yachties seems to be to use a remote sense for the alternator regulator (an easy mod in most cases - or even a standard option) and even a temperature sensor. Whilst using a mosfet rather than a diode will reduce V drop and, hence improve efficiency, the regulation seems to be regarded as more relevant than losing a few extra Watts in the Power chain.
My VSR seems to do the job OK as I haven't been let down by my starter since I installed it last season. (Never let down at all, to be honest - except with the really ancient start battery which had done pretty well over the years.)

There shouldn't be any need for any solid state devices to handle hundreds of Amps because many / most boats are equipped with an A, B, A+B, Isolator switch as an emergency measure.

The economics of fancy regulation when a boat is fitted with PV panel(s) is another hot topic for discussion but the cost of 50W (or so) panels vs MP regulators makes the optimum solution a bit imprecise.

9. May 10, 2014

### jim hardy

Hmmmm... What would Joshua Slocum do?

Is there alink to that VSR relay ?

10. May 10, 2014

### AlephZero

Just make sure your two-battery system fits together better than this:

11. May 10, 2014

### Baluncore

Learn to swim, or disappear in the Bermuda Triangle.

12. May 10, 2014

### sophiecentaur

Slocum had it easy. No one to bump into out there so he didn't need radar.

Quite a character. I read his book a few times. He claims he found an error in the Nav tables and corrected it and, furthermore, he boiled his ships clock, to clean it and then calibrated it - way out in the Pacific, iirc. Very entertaining.

This is the VSR I bought.

13. May 10, 2014

### jim hardy

That relay is a nice gizmo ... better than my oil pressure switch because it'll separate the batteries in case of alternator failure.

14. May 11, 2014

### OmCheeto

I've only two loads on my boat*.
1. Starter
2. Bilge pump (she leaks)

Given that my engine has no alternator, I never go boating without a PV panel.

ps. Slocum's experience with his ship Destroyer, sounds very much like the last day I took my boat out last summer:

I was running on probably 1 out of 4 cylinders, with water up to the gunnels, and my 1100 gph bilge pump running on continuous mode.

Fortunately, it had been a sunny day at the beach, and my panel had kept the "Marine" deep cycle battery fully charged.

pps. In an effort not to get too off topic, anorlunda, listen to these people. Ditch the diode idea, and go with the nice gizmo relay.

ppps. Good grief. I need to listen to the voices in my head:

My immediate solution to the problem:

Purchase a 12VDC to 120VAC converter: $40 Purchase a decent battery charger:$40
Connect the converter to your "bank", and keep your starter motor battery fully charged.
The converter will squeal when the "bank" voltage drops to a certain level.
When the converter starts squealing, consider either investing in about \$100 worth of solar panels, or start the motor.

*When I first saw it, 25 years ago, I thought; "That's a row boat with a motor". Now I see a jet ski that can haul 3 people, and all their junk.