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Battery guards and relays


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Has anyone any experience of "battery guards", ie: low voltage cut-outs?

 

I'm generally fine on the boat, because I'm fairly careful, but running very similar systems on festival sites for a large number of people I've had batteries knackered and I'm looking for a solution.

 

The issue is that I'm running a 1200w inverter and a battery guard that can switch this much current is cripplingly expensive.

 

Many inverters do contain low-voltage cut-outs, although these are usually set very low and not adjustable.

 

It seems to me that one could combine a very low-current (therefore cheap) battery guard with a large relay and have a functioning system for a fraction of the price. Anyone tried this or have any thoughts?

 

eg:

Connect the load terminals of this: http://www.ebay.co.uk/itm/12v-Battery-Discharge-Protector-Battery-Guard-20-amp-/301986489552?hash=item464fcc24d0:g:OEwAAOSwvg9XYZCx

 

To the coil terminals of this: http://www.ebay.co.uk/itm/200-AMP-12V-4-Pin-Car-Automotive-Relay-Switch-ON-OFF-HEAVY-DUTY-SPLIT-CHARGE-UK/162073899676?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D37450%26meid%3D1a207d207e8d4cec9a0dccb951830251%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D321219479424

 

Should create a 150A battery guard (note switching capacity is 150A, not the total rated current of 200A) (appropriate fuse/cb also required).

 

What do you reckon?

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That should work, but the battery guard looks like a simple voltage threshold detector and this might give some problems. What is the inverter driving and how hard is it working? and how much trouble are you in if it spuriously cuts out?

 

A sudden high current demand might activate the cut out well before the battery has got down to what you consider a dangerously low state of charge. You probably don't want somebody trying to use a hairdrier to shut down your fridges etc.

 

...............Dave

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That should work, but the battery guard looks like a simple voltage threshold detector and this might give some problems. What is the inverter driving and how hard is it working? and how much trouble are you in if it spuriously cuts out?

 

A sudden high current demand might activate the cut out well before the battery has got down to what you consider a dangerously low state of charge. You probably don't want somebody trying to use a hairdrier to shut down your fridges etc.

 

...............Dave

 

You make a good point. Ones with separately adjustable cut-out and reconnection voltages exist for a similar price and I'll be sure to get such a one.

 

Edit: no particular problem if it cuts out, it's getting used for phone/drill battery charging with a separate cut-out (set lower) for lights.

Edited by sassan
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I reckon that so-called 200A relay would probably melt.

 

Take a look at this page http://www.smartgauge.co.uk/more_relay.html

 

Tony

Edited by WotEver
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This is where the CE markings come into their own. The standard would state the current and duration before a relay could claim to be rated at that value. BUT, how the general public knows which CE numbers to look for is a mystery, apart from Google of course !

 

Not wishing to get political, but the British Standard kite mark may be making a come back and replacing the CE mark !

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That should work, but the battery guard looks like a simple voltage threshold detector and this might give some problems. What is the inverter driving and how hard is it working? and how much trouble are you in if it spuriously cuts out?

 

A sudden high current demand might activate the cut out well before the battery has got down to what you consider a dangerously low state of charge. You probably don't want somebody trying to use a hairdrier to shut down your fridges etc.

 

...............Dave

Yes Dave's points are valid (of course!) in that instantaneous voltage alone is not a good guide to when to disconnect the batteries. A Smartgauge would be a better option since it could be used to disconnect the relay at a specified actual SoC. But don't forget that to keep a normal high current relay closed, takes significant power 24/7. Unless you can use a bistable relay (ie one that only needs power to change state). You can get remotely operated battery Isolators like that. Edited by nicknorman
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If its ok with the OP can i ask the question if my Victron is safe and not frying the battery bank.

 

I am plugged in on shaw with a tail end charge of 1%

 

1 SOC 100

2 volts 13.23

3 00 A

4 0.00 Amps

 

Am i nackering the batterys into a fizzy frenzzy ?

 

 

 

 

 

Op : if i am naffing up your thread just tell me its cool.

Edited by Greylady2
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