So, diode splitters with battery-sensed alternators?

[prv]

Ariam is getting an electrical system overhaul this winter. Hitherto I've been a fan of VSRs to link batteries while charging and disconnect them when not. However, I noticed the other day that the alternator on our D1-30 has a sense wire running to the service battery, and I guess this means it will compensate for the drop in what I assume is a diode splitter (box with appropriate connections and lots of heatsinking).

Any comments on how this compares to a VSR system? With shore power only rarely applied and a boat that likes to sail, my main goal is to charge up a fair-sized service bank as quickly as possible from limited engine running.

Cheers, Pete

 

[sailinglegend420]

...Any comments on how this compares to a VSR system? With shore power only rarely applied and a boat that likes to sail, my main goal is to charge up a fair-sized service bank as quickly as possible from limited engine running....

Split Diodes were the first automatic solution but they have a voltage drop of 0.7 volts across each diode, maybe double this at high charge currents. This causes a huge power loss, but this loss can be compensated for by having regulators that sense the service battery voltage and boost the charger output. This can produce the right voltage at the service battery but can produce too high a voltage on the starter battery. Consider a heavily depleted large service bank - and an almost full smaller starter battery. The voltage drop across the diode feeding the service bank increases with the current so may well reach 1.4 volts or more. The alternator senses the voltage at the bank and raises the output by 1.4 volts to say 15.8 volts. The voltage drop across the diode feeding the starter bank is only 0.7 volts because the smaller service bank is taking a much much smaller current. So the starter bank is sitting at 15.1 volts for maybe several hours! This is almost an equalizing charge - so not very good for any sealed, Gel, and AGM batteries. Modern split diodes use newer technology with virtually zero voltage drops, but they are much more expensive.

A VSR puts the same voltage on both batteries.

Also I'm sure you are aware that a battery will never get to 100% charged if you never connect to shorepower.

 

[halcyon]

Ariam is getting an electrical system overhaul this winter. Hitherto I've been a fan of VSRs to link batteries while charging and disconnect them when not. However, I noticed the other day that the alternator on our D1-30 has a sense wire running to the service battery, and I guess this means it will compensate for the drop in what I assume is a diode splitter (box with appropriate connections and lots of heatsinking).

Any comments on how this compares to a VSR system? With shore power only rarely applied and a boat that likes to sail, my main goal is to charge up a fair-sized service bank as quickly as possible from limited engine running.

Cheers, Pete

Two reasons behind the VSR was to avoid volt drop in the diodes limiting recharge level plus power loss, simply if dropping 1 v across the diode while charging 100 amp you loosing 100 watt or 7 amp of alternator output. A second reason was to maintain alternator connection to engine battery, so any problem in the splitter keeps alternator connection to engine battery.

What you need is to optimize recharge cycle, fitting voltmeter and ammeter allows you to monitor for max charge voltage and low charge current to service bank, this then becomes full charge. Running longer will give very little capacity, shorter will not fully charge the battery and generate sulphation and surface charge. The ammeter and voltmeter will also give guide to usage levels.

Above that depends on the depth of your wallet.

Brian

 

[jac]

If it's maximum output from engine running then consider larger alternator and or smart charger linked to the alternator- I went with sterling but other makes available as they say.

Not finished the installation yet so can't comment on the practice but in theory the sterling a to b charger sends the un boosted charge to the start battery and then boosts the charge to the domestic bank only. When off it acts as a vsr. Can also take input from solar panels, wind etc.

 

[Richard10002]

What you need is to optimize recharge cycle, fitting voltmeter and ammeter allows you to monitor for max charge voltage and low charge current to service bank, this then becomes full charge. Running longer will give very little capacity, shorter will not fully charge the battery and generate sulphation and surface charge. The ammeter and voltmeter will also give guide to usage levels.

Brian

Hi Brian,

For a particular size of battery bank, how do you know at what low level of current draw the bank is as good as full?

Some references suggest 2% of bank size at 14.4V, but there doesn't seem to be a definitive reference.

With my supposed 330Ah bank, 2% would be about 6 or 7 Amps, and I often run things until its at about 3 or 4 Amps, so I'm hopefully all is well. I have a NASA BM2 monitor, which I am certain is wired correctly.

However, even with the above regime, I usually find that fewer Ah are put back, than have been used..... e.g. I might use 75Ah over a day and night then, when charging the next day, I will reach the low level of 4A input, but will find the NASA BM2 still reading 11Ah discharged. I always zero the reading after charging to what is purportedly full.

It was always my understanding that you had to put back more than you use to overcome losses due to heat etc.?

Any ideas?

 

[prv]

If it's maximum output from engine running then consider larger alternator and or smart charger linked to the alternator

I already have a pretty large alternator - the standard on the D1-30 is 115 amps. As for smart regulators/chargers, I'm a little dubious about what it is they actually do. Certainly the older ones merely boosted the alternator voltage a bit - but modern marine alternators already have that effect built in.

Thanks SailingLegend for the point about overcharging the engine battery - I hadn't realised that the volt drop across the splitter would be different for the two batteries. That sounds less than ideal as I'm planning to use AGM for everything and I believe they're less forgiving of excessive voltage.

Brian - obviously I knew that the splitter would involve a loss of energy - that much is clear from the heatsink fins. I hadn't realised how much, though - 7amps sounds like a hell of a lot. Though of course I doubt it would actually spend much time charging at 100 amps - doesn't the current drop quite quickly as the battery charges up? And is 1v a realistic figure for the drop across a reasonably modern splitter?

We're moving the boat from Hamble Point back to Southampton in the New Year after having some work done. I'll try to remember to measure various significant voltages as we do so, to establish how well the current system works.

Pete

 

[jdc]

...I hadn't realised that the volt drop across the splitter would be different for the two batteries. That sounds less than ideal as I'm planning to use AGM for everything and I believe they're less forgiving of excessive voltage....

It may not be that great a difference in practice. The diodes - provided you've fitted big enough ones - have little series resistance and the current-Voltage relationship is very nearly exactly I = Is exp(eV/kT) as the batteries approach full and thus are nearing maximum Voltage (which is where the delta V between diodes might start to matter).

kT/e is about 26mV at 300K so each decade current ratio between the two diodes gives rise to about 60mV difference in Voltage drop

(delta V = 26mV x ln(10)).

Taking this in practical terms, you may have 35A or so of charge into the house bank, but only leakage into the starter. Self discharge is ~5% / month. That's 70uA / Ah, so a 50Ah battery will discharge at 3.5mA ish, hence a current ratio of 10000, ie 4 decades so 240mV. And to mitigate this effect even more, as the house bank is getting near full the current diminishes so the Voltage on the starter battery will fall.

As a check I measured my system: 14.40 on the house bank and 14.55 on the starter battery when the alternator was supplying a few 10s of Amps and the Adverc was on the 14.4V, ie high Voltage, part of its charge cycle.

Whether this is acceptable for you depends on your set up. I have lead-calcium sealed batteries for the main bank, max Voltage at 25C given as 14.4V, and an Odessy AGM one, max Voltage 14.8V, for the starter. So the difference across the two diodes in unlikely to cause a problem for me, and indeed it hasn't yet after 1500 hours of motoring in 9 years of ownership, all on the same batteries.

So in short, I've had no issues, have well charged batteries and long battery life from a system with diodes and a battery sensed alternator regulator. Which is not to say that VSRs may not be just as good or better, but a well designed diode splitter system is perfectly satisfactory.