Voltage Sensitive Relay Advice Needed. What am I missing?

moosey_fate

New member
I need help to safely charge my house battery off my alternator when my van is running. At issue is the fact that I have a 220A alternator, and I'm having trouble finding an intelligent relay that can handle 220 amps. Furthermore, it looks like I need 4/0 gauge wire to handle a 220A load over a 25' run. Is that accurate?

My understanding of things:
1. A Voltage Sensitive Relay should only be used to connect batteries of the same type and age, so it's not really desirable if I want to use AGM deepcycle batteries in my house.
2. A 220A alternator means that 220 (ish) amps would be going down the wire from my starting battery to my house battery when the relay is connected.
3. A DC-to-DC charger would be optimal for charging my AGM house battery from my starting battery/alternator.
4. There are no DC-to-DC chargers that are rated for 220 amps.

The fact that I'm seeing everyone on Youtube use the 140A Voltage Sensitive Relay with 4 gauge wire tells me I am missing something. What am I overlooking? How can I set up my house batteries to charge off my alternator?

Thanks!
 

OrioN

2008 2500 170" EXT
The maximum amperage that the alternator will supply to your house bank will be under 100A, and in the 40-90A range depending on what the size of the bank is, its SOC (state of charge), and engine rpms.
 

moosey_fate

New member
Thanks for the response! Why is the amperage so low given the alternator rating of 220A? Where are the other 130 - 180 amps going? And if the starter battery is fully charged, why aren't all 220 amps going to the house battery bank?

Also, should I worry about connecting a 200AH AGM battery bank to a 95 AH starting battery?
 

ECU

2006 T1n 118 Sprinter
I'm sure that there will be lots of bigger/better solutions.
The amps pulled to the battery bank will only be what it can take, how low the charge. Your relay will work fine since you're likely to only get 50% down on your batteries.
If you ever looked at the alternator, it has a smaller than 4G wire for charging.
You'll be fine with the standard relay. If you want a DC-DC charger, the rate it charges will be in the 25 amp range.
 

borabora

Well-known member
What is the size and type of your house battery? That answer will decide what the maximum charge current should be. In many cases a battery can draw more charge current than it should. Some battery configurations could charge at more than what your alternator is happy to supply (the rest of the van requires some power as well and a 220 amp alternator is not designed to supply 220 amp continuously over a long time).

Your battery has a max charge specification you should find out what it is. That's the ceiling but charging at a lower rate is fine if you are happy that it takes longer to charge the battery. If you have multiple batteries is parallel then you multiply the max by the number of batteries you have configured in parallel.

A DC-DC charger is generally the best solution because it can charge your battery based on your battery's type and state of charge. The alternator cannot do that. It does have the capability to adjust voltage based on load and time (apparently) but it does not have a way to know what your battery specifically wants (bulk, absorption, float) vs what your starter battery wants.

I own the 20 amp Renogy DC-DC charger and it works fine for my needs. The 40amp models should work fine for you unless you have a battery that must be charged at a lower rate. Otherwise your configuration of 4 gauge wire and a 50 amp breaker sounds good. A 25' wire run is not ideal because of loss due to voltage drop but it is acceptable.
 

elemental

Wherever you go, there you are.
Thanks for the response! Why is the amperage so low given the alternator rating of 220A? Where are the other 130 - 180 amps going? And if the starter battery is fully charged, why aren't all 220 amps going to the house battery bank?

Also, should I worry about connecting a 200AH AGM battery bank to a 95 AH starting battery?
A 12VDC 220 amp alternator is capable of supplying up to 220 amps at 12VDC, but only if the load on the electrical system is drawing that much current. For example, in your house, you have AC power with branch circuits that can supply up to 15 or 20 amps (of 110VAC power assuming you are in the US). I'm sure you don't worry about plugging in a 60 watt light that only draws a little over 1/2 amp, right? You could plug a 1500 watt heater into the same outlet and it would draw 13.6 amps. However, if you plug a big motor that uses 3000 watts, it's going to try to draw 27+ amps and pop the breaker.

Ideally you want to recharge your 200 AH AGM battery at the rate specified by the manufacturer. The generic way of understanding the charge rate is using "C" where 1C amps = the amp-hour rating of your battery, or 200 amps for a 200 amp-hour battery. If the manufacturer says to recharge at .2C, that would be .2 x 200 = 40 amps; if the manufacturer says .4C, that would be .4 x 200 = 80 amps. That recharge rate would be the best for the longest cycle lifetime for your battery (highest number of charge/discharge cycles). Here is an article from "Marine How-To" that discusses charging AGM batteries: <https://marinehowto.com/how-fast-can-an-agm-battery-be-charged/>. There is a lot of other great electrical stuff on the same site. It's focused on boats, but just about all of the electrical stuff can be applied to vans as well.

You have to calculate wire sizes (thickness) for various circuits based on the current flow and the length of the wires (wires have more resistance over longer distances, or if they are smaller - more resistance creates more heat, and more voltage drop). Blue Sea Systems has a wire size calculator that a lot of people use to help them figure out the right wire sizes. You also want to protect wire runs with some kind of circuit interruption device that will pop or blow before you exceed the wires capacity, in order to keep the wire from getting so hot that it catches on fire.

Although a *good* DC-DC charger is an ideal way to recharge your house battery, a bad one can be worse than an automatic charge relay. I'm using an automatic charge relay right now (from Blue Sea Systems), although I plan to install a DC to DC charger soon. Blue Sea Systems has a magnetic latching automatic charge relay (the magnetic latch keeps the relay from having too high a current draw to keep it open or closed) that can handle up to 500 amps of current flow. It senses voltages (on either side) to determine whether to close or not. It can also be operated manually in some versions, or through a remote switch. <https://www.bluesea.com/products/76...rging_Relay_with_Manual_Control_-_12V_DC_500A>. There are lots of trade-offs picking electrical solutions!
 

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