Voltage drop through isolator

[freeheel4life]

Been chasing some electrical issues on a boat or two lately and reading up on others issues here and wondering if anyone can answer a question about isolators. Seeing this mostly on older style three post isolators with cooling fins, usually blue or black. Don't know make or model but Tige liked to mount them at transom.
The question is why is there anywhere from .7 to 1 volt drop from alternator output post to batteries?? Is this normal?? Boat I was working in recently is putting 14.2v out at alternator, 14.1v at isolator post(can accept .1v drop between alternator and post.) But when taking reading at posts from isolator to battery I am getting a .7v drop. Reading up on other electrical gremlins others are seeing a 1v drop. Why?
Batteries are still charging at around 13.5 which is on the low end of acceptable in my mind when full fielding. No change with rpms. Switch on a bunch of loads to try and get the alternator field current up and just experienced same voltage drop thru isolator. What is up??

[chpthril]

They are a diode type isolator, so yes, a voltage drip up to .7V is normal. In most cases, if the alt is strong and there are no other issues like a failing battery or bad or corroded cable end, that .7V creates no issue.

If it becomes a liability, the best fix is to convert to the blue sea dual circuit plus switch and ACR/VSR.

[freeheel4life]

Thanks Mike. In one case it was certainly my recommendation that battery was the cause(low quality centennial brand.) Was getting 13.6 when cable was unhooked from house batt then dropping to 12.8 only when hooked to battery. OCV was almost exactly 2v low. Advised shorted cell, couldnt confirm as another person at shop left our battery tester in a customer boat and is gone now . It's a 12 RZ2 and his stereo is progressively sound worse and worse thru day as he is driving it hard. Told him voltage is going low, needs new battery and recommend new cable from isolator to battery. Customer declined. Cant wait when all 4 icon 8s die due to underpowered with volume maxxed. Cant fix stupid.

[IDBoating]

The question is why is there anywhere from .7 to 1 volt drop from alternator output post to batteries??

To answer your question: 0.7VDC is the typical forward voltage drop of a standard PN silicon junction, of which a typical silicon rectifier/diode is an example. You can use a Schottky diode (more $$$) to get down to around 0.3VDC, but in high current applications such as this the loss of 0.7V isn't usually considered a problem.

That voltage drop does generate waste heat, though. For example, if you're moving 50 amps through a 0.7VDC drop, the result is roughly 35 watts of waste heat. That's why isolators usually have cooling fins, to dissipate the waste heat that results from large amounts of current passing through the various diodes inside.

I've toyed with the idea of designing a more efficient isolator based on PWM-controlled MOSFET's to get away from the PN junction inefficiencies. I've also considered it might be useful to intelligently prioritize battery charging, i.e. sense if the starting battery is low and preferentially route available alternator current there to top off the starting battery quickly before worrying about the house battery, rather than blindly sharing all available current across all connected batteries. But I haven't researched the market and suspect others have already done such things so why reinvent the wheel?

[IDBoating]

It's a 12 RZ2 and his stereo is progressively sound worse and worse thru day as he is driving it hard. Told him voltage is going low, needs new battery....

In other words, he's driving the amplifier outputs into clipping as the voltage rail drops. It's not just the batteries he's putting at risk... he's also risking his amps and speakers. The RMS of clipped waveforms is substantially higher than the unclipped signals, a lesson he may painfully learn if he's not careful. Frankly, I'm surprised today's amps don't detect and prevent this condition but if he can actually hear the sound degradation then it's pretty obvious they don't. Might be interesting to put a scope on those amp outputs, but as you say - you can't fix stupid and the owner probably wouldn't understand what he was seeing anyway.

[chpthril]

I'm surprised today's amps don't detect

There are some that do detect. Rockford has has a line of amps with a clip detection light and Kicker's brand new KXMA line has the detection. You can even get clip indication on the new wireless sub level remote that comes with their mono amps. As to rolling back the output once in clip mode, can answer that one. Most use the clip indicator as a tuning tool as they hope that the system stays stable for the most part, except an occasion burst of clip here and there.

But you are right, the amps output in a state of clip is unpredictable and erratic. Its output can easily reach its peak and exceed the thermal capacity of the woofers voice coil. Wont take long to blow a driver.

This also does not even take into account that the amp is ramping up its current draw as voltage sags, trying to maintain its output. Heat goes up big time. Efficiency drops and sound quality certainly falls off. This is why some amps like the wet sounds, have power roll back based on voltage. Keeps the amp more stable as supply drops.

Ive helped a few tige owners convert from the diode isolator to the DCP switch and an ACR. Well worth it. The bigest flaw in that tige setup when having all the house loads battery direct. I cant count the number of tiges in that era with dead house banks due to a parasitic draw. Putting in the dual circuit plus switch in place of the old on/off, made a huge different in battery life.

[freeheel4life]

Thanks fellas. Had a feeling you two would be my guys on this one. Not the greatest audio guy but pretty competent with a meter and am always concerned when I see voltage drops like that, especially when there's no obvious signs of corrosion/bad connections.
Mike, you nailed it with all functions pulling from house. When taking reading with amps pushing hard you could watch voltage drop lower as other loads we're switched on. Will try and steer towards ACR and some new Batts when he comes back, because I know he will be.

IDboating- Assuming by PWM you mean pulse with modulated. Curious how/why that would benefit. Sorry this is turning into nerd speak, but I love this stuff.

[IDBoating]

IDboating- Assuming by PWM you mean pulse with modulated. Curious how/why that would benefit. Sorry this is turning into nerd speak, but I love this stuff.

Briefly:

Diodes, as noted above, have a voltage drop that makes them inefficient. They're also "stupid", in the sense that they always behave the same way. That's great for many applications but when you're dealing with high(er) currents and waste heat sometimes it's nice to eliminate that inefficiency and also do things smarter.

FET's have become amazingly good. Their on resistance is vanishingly low which allows them to have incredibly low losses when operated fully saturated. That's why Pulse Width Modulation (PWM) is popular... it can allow the transistor to "simulate" linear operation while keeping the semiconductor in its efficient fully-off or fully-on modes instead of the analog "middle of the curve".

If you just want a simple switch, the FET can be quite a bit more efficient than the diode. However, FET's can only block conduction in one direction (due to the inherent body diode) so you can't just swap in a FET for a diode and magically get lower losses; you won't get the isolation that (in the case of a battery isolator) you're looking for. You need some control circuitry to manage the FET's operation.

And once you have control circuitry, lots of things become possible - such as the intelligent battery charging I mentioned. I would MUCH rather be certain my starting battery is fully charged than both my starting and house batteries be half-charged! But a diode-based isolation scheme cannot make intelligent decisions like that... the diodes just share the current from the alternator.

Thus an intelligent "isolator" would give you the ability to reduce losses AND make decisions about prioritization. PWM control of the FET's would take that a step further and allow more nuanced decisions, such as "when the starting battery reaches 90% charge, start channeling a rising percentage of the available charging current to the house battery". Or "The longer the engine runs in a given cycle, the safer it is to route increased charging current to the house battery". The unit could even be made heuristic, learn about your usage patterns over time and adjust its charging behavior based upon a combination of battery state(s) and engine conditions. A cold engine, for example, is more likely to be left running longer (you're probably warming up the engine) but the engine temp has no effect on alternator output, so you could make different decisions. That sort of thing.

Does that help?

[freeheel4life]

It does. A lot to chew on for a guy with a diesel/heavy equipment background gone boat tech, but used to talking about PWM and CAN as it's old tech getting used across the market on EVERYTHING. Definitely entering the realm of electrical engineering here though. Really like the idea of learning usage patterns, particularly as amp draw for stereo would increase and decrease with speed(assuming alternator current would be prioritized during periods of high usage aka surfing with tunes cranked). Going to have to do more homework on FETs as solidd state devices/semiconductors are certainly not my specialty, though have a basic understanding. Thanks for dropping some knowledge on me. Truly appreciate having more to digest.

[ericinmich]

I didn't work directly on them, but new some in another group at my OEM that did the battery SOC algorithms, which were done a decade or so ago before much hybrid work was going on. To do SOC estimation (only can estimate through models, no direct measurement) there is a lot more than one would think. And to deal with old batteries and then offboard charging and battery swap logic, it can get out of hand. Cars now will not charge the battery much until decels then hit them hard saving a bit a fuel.

I think it's BMW that has an algo that needs to be reset/relearned with new battery, and a battery swap now costs a few hundred dollars. They also had a recall as they torched a few cars due to the algo not working well.

I can't see how it would be worth it on lead acid batteries. Maybe when we all have Li batteries in boats, something smarter could be done.

[IDBoating]

I can't see how it would be worth it on lead acid batteries. Maybe when we all have Li batteries in boats, something smarter could be done.

Traditional lead acid batteries don't give access to the inter-cell connections so you can't/don't do cell balancing. You can only charge the entire *battery* (using the proper definition of the word) as a single unit from its "outside" terminals. As a result, the algorithms aren't exotic.

But I was focusing on selective charging of the different batteries on the boat. In most cases it's more important from a safety standpoint to charge the starting battery
than the house battery, so it would be nice to favor the starting battery and then hold it at a trickle charge while charging the house battery.

Battery isolators, "smart" relays, etc. aren't doing any fancy charging algorithms. They're just using diodes to parallel charge the batteries, sometimes selectively paralleling them after they're all "charged enough". I think it can be done better, in fact I suspect someone must have already done it.

[ericinmich]

Traditional lead acid batteries don't give access to the inter-cell connections so you can't/don't do cell balancing. You can only charge the entire *battery* (using the proper definition of the word) as a single unit from its "outside" terminals. As a result, the algorithms aren't exotic.

But I was focusing on selective charging of the different batteries on the boat. In most cases it's more important from a safety standpoint to charge the starting battery
than the house battery, so it would be nice to favor the starting battery and then hold it at a trickle charge while charging the house battery.

Battery isolators, "smart" relays, etc. aren't doing any fancy charging algorithms. They're just using diodes to parallel charge the batteries, sometimes selectively paralleling them after they're all "charged enough". I think it can be done better, in fact I suspect someone must have already done it.

I understand what you are saying, but to selectively charging them would take knowledge of what their current SOC is, at least that is what I understand you mean. That is harder than it seams.

[IDBoating]

I understand what you are saying, but to selectively charging them would take knowledge of what their current SOC is, at least that is what I understand you mean. That is harder than it seams.

There is a lot of history handling lead acid cells. In fact, I've had a tech manual from Eveready on my shelf for years that goes into more depth on battery chemistry and charging tech than I ever knew existed.

The current SOC can be monitored in real time if you have the circuitry to do it. Not complicated.

The problem with simple battery isolators is that they DON'T give any consideration to proper charging. They just dump current at the battery. But those advanced lead-acid chargers/maintainers are much more sophisticated, using multi-step algorithms (constant current, then constant voltage, then steady voltage, while monitoring the battery voltage along the way). An advanced battery isolator could incorporate such algorithms and thus both improve your safety (by favoring full charges on your starting battery first) and the lifetime of those expensive batteries too.

[chpthril]

There are some distribute on demand isolators out there, but I dont think they work as well as a more simple one like an ACR/VSR. 2 issue. one, alternators are not really designed as battery chargers. They are not intended to evaluate, recharge and condition. All we want from an alternator to do is supply enough to replenish whats used by normal loads and keep pace with normal demand. SO as long as the regulator is do its job and not overcharging, we just need to supply voltage/amperage based on load, and not worry about the condition of the batter(s).

With the right switch and ACR, charge priority can be given to the cranking bank. Heavy load on the house side and it gets isolated from the main cranking. Smarter then a diode setup and no voltage drop, but simple enough to not impede a stupid alternator voltage regulator.

A perfect world would be a 2 post alternator with distribute on demand logic incorporated. Basically a belt driven on-board 2 bank charger.

[IDBoating]

A perfect world would be a 2 post alternator with distribute on demand logic incorporated. Basically a belt driven on-board 2 bank charger.

That's basically what we've been talking about, except that the electronics would be within the alternator housing.

I'll go one step further and suggest that the perfect world would include the ability to specify the battery chemistry, so that you could use the proper charging and maintenance algorithms for lead acid, or AGM, or gel cell. Each has its preferred algorithm (my home chargers allow you to select), none of which is "raw alternator driving through a diode" {grin}. 12V automotive batteries are available in all three chemistries and it would be nice to support all of them properly.

I'd prefer to keep the alternator and electronics separate, if only so you can mix-and-match to suit your requirements or desires. But no matter how you package it, I think there's an opportunity here for improved safety and battery life. I feel like I'm talking myself into doing it!