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Author Topic:   Voltage Sensitive Relay
jimh posted 09-28-2005 08:55 AM ET (US)   Profile for jimh   Send Email to jimh  
A modern approach to charging two batteries in an outboard powered boat from a single charging source uses a Voltage Sensitive Relay (VSR). This device connects a secondary battery to the primary battery and charging current source whenever the voltage of the primary battery exceeds about 13.2-Volts DC. In this way, the secondary battery only receives charging current after the primary battery has been charged to a voltage which normally indicates full-charge has been restored in the primary battery .

The philosophy of this type of wiring arrangement is to concentrate first on charging of the primary battery, which is usually the engine starting or cranking battery, and only provide charging current to the secondary or house battery when the primary battery has been fully re-charged. In this way the system assures that the engine starting battery will always be kept at a high charge level, and charging of the house battery will never divert current needed for the starting battery.

The manufacturers of VSR devices caution that capacity of the secondary battery and the charging current available need to be matched up. This is necessary to prevent the system from going into an oscillation. Here is what can happen if the charging current available is too small and the secondary battery is too large:

When the charging current has raised the voltage on the primary battery to a level above the threshold of the VSR, the secondary battery is connected in parallel. Typically the addition of the second battery will cause some drop in voltage in the charging current output and primary battery. If this voltage drop is excessive, it will fall below the threshold of the VSR and the device will disconnect the secondary battery. Once that happens the voltage rises, again, and this pushes it above the the threshold of the VSR. The two batteries are connected, and the voltage drops. This sets up an oscillation or chatter of the VSR.

To prevent this the manufacturers incorporate some hysteresis in the control loop of the VST, so that the on-threshold and the off-threshold are separated by a margin of appropriate voltage. However, if the secondary battery is too large and the charging current source too small, the system will still oscillate or chatter, in spite of the hysteresis in the control loop. This can occur if the secondary battery is very large and the charging current is marginal.

I am curious if we have anyone using a VSR in their dual-battery single-charging source boat electrical system. If so, I would be very interested to hear if any problems were encountered with the VSR chattering when the secondary battery comes on-line for charging.

jak posted 09-28-2005 11:06 PM ET (US)     Profile for jak  Send Email to jak     
I'm using the BatteryLink Automatic Charging Relay (ACR) that Blue Sea Systems introduced earlier this year on my Guardian 18. So far the ACR operation has been flawless. I have had no indication of chattering. I'm using two group 27 batteries. One is a standard wet cell starting battery and the second is a AGM deep cycle used as a house battery. What I like about the unit is both the open and close voltages and the high voltage cutout are adjustable/ It has low current draw and a low-voltage disconnect. The unit also has an LED that indicates when the primary battery is being charged or when both batteries are being charged. It also has the capability to remote the LED as well as a remote on/off capability. I'm using two voltmeters mounted on the console (one wired across each battery using separate Blue Sea battery switches) to visually monitor the charge going to each battery. Charging source is the single alternator on a 150-HP Yamaha carburetor motor. I also installed an onboard battery charger wired into the ACR but haven't had to use it so far. The alternator through the ACR is keeping both batteries fully charged.

Picture 1

Picture 2

jimh posted 09-30-2005 10:19 AM ET (US)     Profile for jimh  Send Email to jimh     
Here is the operating manual for the ACR:


andygere posted 10-01-2005 01:54 AM ET (US)     Profile for andygere  Send Email to andygere     
I installed a BEP Marine switch cluster, which has an integral VSR. I have identical Group 27 dual purpose batteries. The only time I have had the relay chatter is after starting the main engine and running at slow idle when the house battery was very weak. This was easily corrected by increasing the main motor to fast idle, thereby bringing up the charging current a bit. Overall, I'm very pleased with the system.

I read recently in a review of the V6 E-TEC motors that a VSR is built into the charging system, which would be really neat. I'll look for the article.

jimh posted 10-04-2005 12:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Andy--Thanks for that image of your installation. I think the BEP cluster makes a very neat arrangement of switches and VSR, with a minimum of additional wiring needed.

My only suggestion would be to make a splash shield so that water cannot enter the switches from the top.

I have been giving some thought to changing over to a set up likes yours, but it is down a few rows on the project list at the moment.

andygere posted 10-04-2005 12:58 PM ET (US)     Profile for andygere  Send Email to andygere     
Thanks for inserting the photo into the thread. You read my mind on a splash shield or some other type of waterproofing. I am considering making a foam backing around the cables, sealed with a carefull application of silicone caulk.

A nice feature of the BEP cluster is that there are removable ports on all sides of the switch clusters that allow you to route your cables any way you wish. The cluster is also available in a linear arragnement, with all four units in a row. I spent quite a bit of time fussing with the arrangement of the cables before I completed the installation, since one of my goals was to clean up the rat's nest of wires that were residing back there. Also note the electric downrigger outlet, which uses a nifty trolling motor plug set into a custom teak mount. Just out of sight is the 30 amp breaker on the house wiring system, which is tucked under the gunwale in its own custom teak mount.

Since installing this system, I haven't had to charge the batteries with a portable charger. Before doing the work, I had to do that at least once or twice per season. I do have some heavy house loads, including radar and a large halogen floodlight on the deck, which I use when cleaning up the boat after evening runs.

karasu posted 02-26-2006 06:35 PM ET (US)     Profile for karasu  Send Email to karasu     
I found your article very interesting. I am looking at installing a BEP marine battery cluster switch in my 21 ft. boat. I run a Yamaha 150 4 stroke, and have a Yamaha 9.9 4 stroke as a trolling motor. 99% of the time I am running one or the other motor, but occasionally I will run both at the same time to warm one up, or flush one while the other is running.

BEP Marine makes a single engine battery switch cluster and a twin engine battery switch cluster. For my application, do you think I need the single engine cluster or the twin engine cluster.

I have emailed BEP directly in New Zeland, but I have not received a response yet.

Any suggestions would be greatly appreciated thanks, JR.

karasu posted 02-26-2006 06:37 PM ET (US)     Profile for karasu  Send Email to karasu     
I might add, that I already have two batteries, and am currently using the standard circular four way battery switch with "off," "1," "both," and "2" positions.
andygere posted 02-26-2006 11:34 PM ET (US)     Profile for andygere  Send Email to andygere     
I have a kicker motor that is wired to the house side of the cluster. I do not run the engines at the same time because I believe it is possible to damage the charging systems if both motors are running and the VSR is closed (the batteries are in parallel). I'll warm the kicker up a the dock before running the main outboard, then the kicker fires right away when I need it. I considered the twin engine set-up, but decided it was overkill for what I wanted to do. If you desire to run both engines at the same time, it might be worth considering.

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