DiploStrat
Expedition Leader
So, what is wrong with solenoid/relay systems? Assuming that you are starting with adequate voltage (call it over 14v at 70F) and adequate current (call it over 100A) not much, if, and this is a big "if" you are careful in your installation.
The typical RV set up looks something like this:
-- 6 AWG running from a starter battery to a camper battery(s).
-- Key controlled solenoid.
In this scenario, it is unlikely that you will ever achieve good charging performance because the wire (and possibly the relay contacts) will introduce too much voltage drop. This voltage drop causes three problems:
-- Most obviously, the voltage may be too low to charge the battery, and,
-- If the voltage is too low, then the current flow will be too low.
-- But perhaps the greatest problem is that the engine alternator and regulator will never "see" your camper battery and thus, as soon as the starter battery has recovered from engine start, the charging system will cut way back to its "float" setting.
This is one reason you hear so many folk saying, "Drove all day and the batteries never got charged." (Why do people accept this poor performance? Mostly because they don't know it could be better and they plug into an RV park every night. My criteria were for a vehicle that could be used for extended periods outside the U.S. without ever needing to be plugged in. At this point, I am going on one month without ever using shore power, either when living in the camper or when it is parked.)
An A2B or a B2B can address this problem as they use their diodes to produce a voltage drop in the starter battery, causing the alternator to ramp up the production of current. They then boost the voltage to meet the needs of the camper battery. As noted, these beasts work as advertised, if the output of the B2B is a bit low.
But once you realize that for any of these systems to work well, you need BIG cables, starting at, say, 1/0 and going up from there, several good things start to happen.
-- Big cables reduce your voltage loss to (hopefully) .5v or less and allow the full output of your alternators to reach your camper batteries. (I routinely see over 150A at idle.)
-- With this lower resistance path, the engine alternator system sees both batteries as one big battery and charges them as such. This avoids the problem of over or under charging one or the other.
-- Replacing a key controlled solenoid/relay with an automatic controller/relay system (Blue Sea ACR, Magnum SBC, SmartBridge, etc.) allows you to take advantage of a charge source connected to your camper batteries, e.g. solar or shore power. This obviates the need for products like the Trik-L-Start (http://www.lslproducts.net/TLSPage.html) which are used to bypass key controlled solenoids or diode systems. For folks in the U.S., probably the best, simplest, but most full featured option is available from Amazon at less than $200. http://www.amazon.com/Blue-Sea-Systems-Automatic-Charging/dp/B001VIXLRO/ref=sr_1_1?ie=UTF8&qid=1394215529&sr=8-1&keywords=blue+sea+ml-acr
Of course, solenoids are electro-mechanical devices and there are two things to watch:
-- The solenoid must be rated for continuous duty or it will overheat and fail, and,
-- The contacts must be large enough to handle the current, typically over 200A. Solenoids of this type, e.g. Blue Sea L or ML (http://www.bluesea.com/products/category/Solenoids) are expensive.
All of the information in these two posts is correct to the best of my knowledge and based on my personal experience. As always, YMMV. And here's hoping that there are not too many typos.
The typical RV set up looks something like this:
-- 6 AWG running from a starter battery to a camper battery(s).
-- Key controlled solenoid.
In this scenario, it is unlikely that you will ever achieve good charging performance because the wire (and possibly the relay contacts) will introduce too much voltage drop. This voltage drop causes three problems:
-- Most obviously, the voltage may be too low to charge the battery, and,
-- If the voltage is too low, then the current flow will be too low.
-- But perhaps the greatest problem is that the engine alternator and regulator will never "see" your camper battery and thus, as soon as the starter battery has recovered from engine start, the charging system will cut way back to its "float" setting.
This is one reason you hear so many folk saying, "Drove all day and the batteries never got charged." (Why do people accept this poor performance? Mostly because they don't know it could be better and they plug into an RV park every night. My criteria were for a vehicle that could be used for extended periods outside the U.S. without ever needing to be plugged in. At this point, I am going on one month without ever using shore power, either when living in the camper or when it is parked.)
An A2B or a B2B can address this problem as they use their diodes to produce a voltage drop in the starter battery, causing the alternator to ramp up the production of current. They then boost the voltage to meet the needs of the camper battery. As noted, these beasts work as advertised, if the output of the B2B is a bit low.
But once you realize that for any of these systems to work well, you need BIG cables, starting at, say, 1/0 and going up from there, several good things start to happen.
-- Big cables reduce your voltage loss to (hopefully) .5v or less and allow the full output of your alternators to reach your camper batteries. (I routinely see over 150A at idle.)
-- With this lower resistance path, the engine alternator system sees both batteries as one big battery and charges them as such. This avoids the problem of over or under charging one or the other.
-- Replacing a key controlled solenoid/relay with an automatic controller/relay system (Blue Sea ACR, Magnum SBC, SmartBridge, etc.) allows you to take advantage of a charge source connected to your camper batteries, e.g. solar or shore power. This obviates the need for products like the Trik-L-Start (http://www.lslproducts.net/TLSPage.html) which are used to bypass key controlled solenoids or diode systems. For folks in the U.S., probably the best, simplest, but most full featured option is available from Amazon at less than $200. http://www.amazon.com/Blue-Sea-Systems-Automatic-Charging/dp/B001VIXLRO/ref=sr_1_1?ie=UTF8&qid=1394215529&sr=8-1&keywords=blue+sea+ml-acr
Of course, solenoids are electro-mechanical devices and there are two things to watch:
-- The solenoid must be rated for continuous duty or it will overheat and fail, and,
-- The contacts must be large enough to handle the current, typically over 200A. Solenoids of this type, e.g. Blue Sea L or ML (http://www.bluesea.com/products/category/Solenoids) are expensive.
All of the information in these two posts is correct to the best of my knowledge and based on my personal experience. As always, YMMV. And here's hoping that there are not too many typos.