what do i need to protect my batteries from getting overdrawn?

parkerkempf

So i'm running 400w of solar on my sprinter van roof to a 35a charge controller to two 6v T105's (in series and everything protected with appropriate inline fuses), from those batts im running a 2kw inverter (with an inline fuse as well) for AC things like charging phones, vitamix, coffee grinder, etc,

also, 

I have a bunch of DC things like fans, fridge, lights, and heaters (like 15 amps if its all going at once)
my plan is to have a fuse box close to the battery, but i dont want all those things to run all night and overdraw the batteries...what do i need to put in between the fuse box and the batteries to protect them from this?

thanks!

westbranch

If they are good Quality DC rated breakers, they can server as disconnects to kill all loads, even the one to the inverter... after the loads are off

jonr

I would consider this alarm.  But I don't see an automated disconnect.

http://www.bogartengineering.com/products/trimetrics/

softdown

I consider the ability to disconnect due to low voltage to be absolutely critical for battery preservation. The regular inverters shut down at 10.5 volts (terrible) or 10 volts (even worse). Batteries left at 10.5 volts may not be long for this world. I have killed several due to 10.5 voltage disconnect default setting.

I was able to find some disconnects for 12 volt and 24 volt systems. Some have housing, some do not. Some are rated for 10 amps, others for 30 amps. What they all have in common, so far, is the belief that we must run small wires to our inverters. I might be able to fit  #8 AWG into the wiring receptacle but probably not without cutting some wires. All you can then do is minimize the distance between the inverter and low voltage disconnect. Or something along those lines.

In the real world, most larger inverters employ #2 AWG, or better, to connect to batteries. At 12 volts and up to 15 amps/120 volt, you should use large inverter cabling. Like #2 or larger...fine stranded welding cable is very easy to work with.

At this moment in time, this is a huge problem. I'm sure we have some expert$ with a viable solution. The expert$ sometimes are of the mind that solar enthusiasts must surely have access to a money tree.

Don't ever let your batteries stay below ~11.5 volts for very long. The premium solar batteries are built to handle periodic low voltage but, then again, replacing them costs three times as much.

I recommend an ~11.5 volt disconnect level. After killing a number of batteries, I consider myself moderately experienced in this area.

BB.

That is a lot of loads... For 2x 6 volt @ 200 AH batteries--I would suggest a ~2500 Watt maximum load (for reliable operation)--A 2,000 Watt AC inverter is probably going to suck those guys down too quickly to be of much use. A 2 kW inverter will take around:

• 2,000 Watts * 1/0.85 inverter eff * 1/10.5 volts cutout = 224 Amps (worst case, maximum 2 kW load)

That is a lot for that small of battery bank. And a 400 Watt array will not supply that much energy. Say ~4 hours of sun (decently sunny area minimum for 9 months of the year):

• 400 Watts * 0.52 typical off grid system eff * 4 hours of sun = 832 Watt*Hours of AC power per day (average minimum, excluding winter).
  

That is about 20 minutes of 2kWatt inverter (running at full load) per relatively sunny day.

-Bill

South Africa

I use a Victron battery monitor, where I set the SOC where I want loads switched off.

Once it hits the set SOC, the BMV's internal relay is triggered.

This little relay is used to trigger a larger AC relay to switch off all loads, connected to the inverter, I want off.

softdown

South Africa said:

I use a Victron battery monitor, where I set the SOC where I want loads switched off.

Once it hits the set SOC, the BMV's internal relay is triggered.

This little relay is used to trigger a larger AC relay to switch off all loads, connected to the inverter, I want off.

That looks like a nice solution and does have blocks to connect good sized lugs. At around $200 it is not cheap.

softdown

http://www.projecta.com.au/Products/DualBattery/LowVoltageDisconnects.aspx

The shipping from Australia seems to be unavailable or prohibitive.
 
Plus the disconnect points are too low or too high for my tastes.

South Africa

Jip they are not cheap but here in SA a lot of us use them. Swapping on voltages just never worked for any of us.

We change power sources between inverter and Utilities, powering a lot of devices in our houses, to save on batteries.

We even wrote software using the data from BMV's to automate it all even more.

But yes, they are expensive.

softdown

Interesting that Xantrex has several charge controllers that are much less expensive than their low voltage disconnects.

softdown

It is not hard to find low voltage disconnects, some from reputable manufacturers such as: http://www.ebay.com/itm/BLUE-SEA-7635-M-LVD-LOW-VOLTAGE-DISCONNECT-/351836956899?hash=item51eb1de8e3:g:6jUAAOSwGtRX1cRO&vxp=mtr

The vast majority are made to disconnect a single battery using small amounts of power. The trick seems to be finding disconnects that can handle ~#2 AWG cables and commensurate amperage draws.

Johann

softdown said:

It is not hard to find low voltage disconnects, some from reputable manufacturers such as: http://www.ebay.com/itm/BLUE-SEA-7635-M-LVD-LOW-VOLTAGE-DISCONNECT-/351836956899?hash=item51eb1de8e3:g:6jUAAOSwGtRX1cRO&vxp=mtr

The vast majority are made to disconnect a single battery using small amounts of power. The trick seems to be finding disconnects that can handle ~#2 AWG cables and commensurate amperage draws.

My inverter got a build-in switch that can be turned off and on manually.
So the trick would be that a low voltage disconnect device would turn the switch on and off at the inverter. So in reality, a low voltage disconnect device could be wired in parallel to that on-off switch at the inverter with very small wires like a #14 etc etc, just make sure that the internal switch/relay of the low voltage disconnect device is a '' dry '' switch ( no actual power coming out on that internal switch).  The # 2 wire would stay where it is.

A while back I bought 2 of those and I used one for about 2-3 month as a temporary charge controller after my MPPT went up in smoke, just add a relay etc to up the amps you can switch. Something like that could turn other devices on or off depending on voltages of the batteries .

I did not use/wire that ebay controller to control my inverter yet, but eventually I will get to do it.
I have seen them for 24 volts and if I remember right I even seen one in a 48 volt version.
if you look around, you even find them with a box/covers.