With two DC charging sources, are blocking diodes required?

[Peter Dill]

TL;DR

I have a simple 12vdc system on a boat with a single battery bank. There is a stock alternator on the engine and i've inherited a solar panel i'd like to install. I understand that most solar charge controllers have built in back feeding prevention, but will i also need blocking diodes on the alternator to prevent destroying the rectifier/voltage regulator?

Thank you

I have a simple 12vdc system on a boat with a single battery bank. There is a stock alternator on the engine and I've inherited a solar panel i'd like to install. I understand that most solar charge controllers have built in back feeding prevention, but will i also need blocking diodes on the alternator to prevent destroying the rectifier/voltage regulator?

Thank you

 

[Baluncore]

Voltage regulators are usually designed to survive high battery voltage while they are not generating.
When operated, a starter motor may produce a voltage spike that could damage the solar regulator.
You might be able to isolate the solar regulator during starter operation by connecting it through an alternative pole of the starter switch.

 

[anorlunda]

will i also need blocking diodes on the alternator to prevent destroying the rectifier/voltage regulator?

No. You do not need a diode to protect the alternator when charging from a charge controller or when charging from shore power. The alternator already has internal diodes.

 

[essenmein]

If you are connecting any electronics to a 12V automotive system you should be aware that the voltage you can be exposed to are maximum 40V for clamped alternators, about 100V for older alternators. This voltage comes from the much loved/feared load dump, for clamped systems the alternator rectifiers are built with zener diodes, and they clamp the alternator output to about 40V during a load dump, if unclamped you'll get the 87V+Vbat (ISO16750, LV124 etc).

Load dumps are very rare (disconnect load/bat from alt while charging at 100% alternator capacity), but they are devastating to things that aren't designed to handle them, since the output of the alternator is able to deliver a lot of current during this pulse.

 

[Peter Dill]

Thanks everyone, massive help. Now I've just got to find the right controller for the panel.
Here are the specs: https://www.kyocerasolar.com/dealers/product-center/archives/spec-sheets/KC40T.pdf
Any insight on selecting a controller?

 

[anorlunda]

Any insight on selecting a controller?

For maximum power, you will want a MPPT controller. Because your panel is very modest size (It is just one panel, correct?), you'll probably be forced into buying something with more capacity than you need.

Don't forget that you will also need a 3 stage smart battery charge controller. If you are able to find both controllers in one package, you might save money.

West Marine usually has very comprhensive choices for marine equipment. I would check there. Marine hardware is also recommended because of resistance to corrosion in the electronics because of the humid, perhaps salt water environment.

In my experience, inexpensive electronics has a lifetime of only 2 years in the salt environment. Seaworthy devices cost much more, as do airworthy devices used on airplanes. If you're crossing oceans, reliability is paramount and expense secondary. If you're a weekend day sailor, it's probably cheaper to buy cheap stuff and replace it often.But you haven't given much detail about your use of power. If you are a weekend sailor who allows the battery to recharge on weekdays, then even 43W is overkill. MPPT, and 3-stage battery chargers are also overkill. You can buy the least expensive controller which can handle 7 amps and uses PWM to limit battery voltage under charge to 13.8 V. Those cost less than $20 at West Marine, probably less from discount sources.

 

[Peter Dill]

Thanks Anorlunda!
You are correct, it's just the one panel. As far as usage goes, i generally live on the boat over the weekends and I have fairly small electrical demands, the largest draw being the refrigerator (which i use as an ice box because of the power requirements). The other loads are just lights, fans and a water pump
I have two 90 AH Gell Cells and the boat lives on a mooring, so there is no opportunity to plug the battery charger in under normal circumstances

PWM certainly sounds like the way to go at the moment. I do have dreams of taking her across the ocean, but these are long terms goals.

Just happened to have one of these kicking around from an old flexible panel that aged and died: calendar.google.com/calendar/r?tab=cc
Not a great quality controller I'm sure, but should get me going.