Looking for the most accurate, highly recommended battery monitor

[dreadlocks]

https://www.victronenergy.com/battery_protect/battery-protect

I use the 100A version, but they also have a 65A and 220A depending on the loads.. it works well all by its self fully automatic if you want simple and idiotproof, or you can use it in conjunction with a battery display so you can monitor loads and charging in realtime... you may not find it usefull, but it does help when your on a limited power budget to know how much power things really use and how much time left you have left in the tank..

for example the fan I have takes like ~1A on low and ~5A on high.. knowing its consumption I dont run it on high for long times as its not worth it, but I do feel fine running it on low all day long.. without this data, my behavior would be much worse on my energy budget..

 

[jonyjoe101]

You want an over/under voltage relay, those are only 6 dollars. I use those to control overvoltage on my lifepo4, but you can use it to disconnect your battery when the voltage reaches 12.1 volts. These are extremely accurate, they measure the voltage from battery terminals, I have mine triggering a larger 30 amp relay but they can also trigger a solenoid in the hundreds of amps.

Doesn't get any easier then that. These relays are extremely reliable, I had them connected 24/7 for years. They always work when they have to. Plus with the LED you always know what the battery voltage is. I highly recommend these, they are very useful relays.

 

[dreadlocks]

automotive relays/solenoids have coils that pull a ton of current just them selves, for a house battery you will wanna switch over to something with a solid state relay so its not draining the battery in its self.. The Victron is a solid state and only consumes 1.5mA engaged and 0.6mA disengaged.. where a continuous duty starter solenoid will draw whole amps more than likely.. even that little one you pictured probably uses 150-300mA.. and then your voltage monitor is a parasitic drain that can crush a battery if left unattended for a long time so you want that to be as minimal as possible.. for a load that is always running regardless if your using anything or not, this can end up hurting your capacity alot if you dont use much in the first place.

For only $51 prime for 65A version, its plug-n-play and simple.. worth the investment if you have any money at all in your batteries.

*edit* some maths with online calculator
Coil Relays: 300mA continuously will consume half of a 100AH lead battery in 7 days with no other loads or charge inputs (2 weeks with 150mA relay)..
Solid State Relay: 1.5mA on same battery will drain it half way in 3.8 years.
source: https://ncalculators.com/electrical/battery-life-calculator.htm

 

[dwh]

Why would a shunt be necessary or an added feature benefit?

Any DC ammeter - except a clamp around inductive type - requires a shunt.

https://www.bluesea.com/articles/552


Digital shunt type battery monitors - Blue Sea, Xantrex, Victron, Bogart - count the amps in and the amps out. You program it with the battery amp*hours capacity and by counting the amps, it figures out how full the battery is.

These are commonly known as "amp counters". But, because the actual capacity of the battery goes down over time, and because it takes more amps in to recharge a battery than amps that went out, amp counters get more inaccurate over time. To keep them calibrated you have to periodically do a capacity test and reprogram the amp counter with the corrected capacity.


Algorithmic battery monitors like the SmartGuage measure voltage and resistance then compare those numbers to known profiles stored in memory. They use an algorithm to determine fullness.

Algorithmics don't need to know amp*hour capacity. They start out a bit innaccurate and get more accurate over time as more datapoints are logged.



But since algorithmics don't count amps, they can accurately tell you the battery's state of charge (SoC), but they can't tell you anything about the amperage input/output. Most people like to know how many amps are flowing in from solar or a shore charger or alternator, and how many amps are flowing out to feed the loads. You need an amp counter for that.

 

[john61ct]

Great information from everyone, Thanks.

What would you say is the lowest voltage you can safely drop to before recharging the battery and damage it? This is for a deep cycle battery. That is what I want the battery monitor to tell me.

Voltage is not irrelevant, but a **very** inaccurate indicator of SoC.

The meters I discussed will serve you much better.

The general **rule of thumb** is 50%.

If you need to occasionally go down a bit more you will not reduce longevity by much.

Some people only discharge 20-30% and that will **greatly** increase bank lifetime

**if** the bank is well coddled otherwise

but then your upfront cost and extra weight carried around is increased.

 

[john61ct]

Yikes. So I've metered my battery at 10.9 volts in the past, and in the 11's several times... Is that battery trashed? Which is why I need an accurate battery monitor.

The more frequently you do that the more longevity will be affected.

But the standard "zero point" is defined at 10.5V, and the most-coddled banks that can last well over a decade may go that low say a dozen times in their lifetime when their State of Health is tested.

The key is to start recharging back to 100% Full **immediately** after hitting the low point.

 

[john61ct]

Any device put in line to monitor or control the current must be rated well above the maximum amps you will put through that circuit.

With known high quality vendors like Victron, Balmar, Merlin etc you can trust their ratings and only give say 10% extra margin.

With cheap Chinese stuff I would not put even half the amps they claim through regularly or for very long.

Most good shunt BMs will give the specs to use a variety of sizes, many find 100A enough, but can use 500A or 1000A when needed.

SmartGauge not being in-line nor shunt based, attaches to any size bank (lead only), and does not care about ampacity.

 

[john61ct]

Great information from everyone, Thanks.

What would you say is the lowest voltage you can safely drop to before recharging the battery and damage it? This is for a deep cycle battery. That is what I want the battery monitor to tell me.

We recently had a very in-depth technical discussion on average DoD vs lifetime cycles over at the Cruisers forum.

http://www.cruisersforum.com/forums/f14/depth-of-discharge-myth-206151-19.html#post2706281

If you come across jargon unfamiliar to you, GIYF, or post followup Qs.

Pay close attention to whatever Maine Sail says, and read all the electrickery articles at his http://marinehowto.com

 

[Dr Gil]

The more frequently you do that the more longevity will be affected.

But the standard "zero point" is defined at 10.5V, and the most-coddled banks that can last well over a decade may go that low say a dozen times in their lifetime when their State of Health is tested.

The key is to start recharging back to 100% Full **immediately** after hitting the low point.

I like simple and I like the idea of the load shutting off if the battery gets to x-point. I feel like the Victron low volt disconnect, and seems to fit my needs. I really don't need a battery monitor like I thought I did. Great ideas here have changed my priorities. I have an Blue Sea ACR, and solar to use when I'm out in the wilderness, so I'm not worried not starting the vehicle. I can then start charging the Aux battery when I'm on the road (or by solar) and get to shore power and use my battery tender.

 

[john61ct]

Yes, if the goal is simply to isolate the battery at voltage X, an LVD device will be enough.

What Victron unit do you mean specifically?

I like my LVDs to be adjustable, for example to shut my freezer down long before the CO alarm.

And just a reminder, voltage is not SoC, a high-current load on a smaller or older bank can drag "surface voltage" down while SoC is still quite high.

 

[Dr Gil]

Yes, if the goal is simply to isolate the battery at voltage X, an LVD device will be enough.

What Victron unit do you mean specifically?

I like my LVDs to be adjustable, for example to shut my freezer down long before the CO alarm.

And just a reminder, voltage is not SoC, a high-current load on a smaller or older bank can drag "surface voltage" down while SoC is still quite high.

The Victron 65 amp was recommended by Dreadlocks. I only plan to put it on my ARB fridge.

 

[john61ct]

That's under 10A load, likely half that

maybe this, fully adjustable
https://www.ebay.com/itm/Low-voltage-Cut-Off-12V-15A/192676355704

Or via Amazon https://www.amazon.com/bed-electronics-SBS1215C2-Battery-Protector-12V/dp/B074VBDBY2/

even cheaper, but likely not as good
https://www.ebay.com/itm/12V-Batter...-On-Recovery-Protection-Board-al/202437447465

 

[dreadlocks]

dont ARB fridges have built in LVD? Mine did before the display konked out.

Even if you have multiple appliances I still believe you should have a master disconnect lvd to ensure your battery drops all loads at the lowest SOC/voltage your willing to accept, a fridge full of steaks is cheaper than a new set of batteries if they drop down to 10.5v.. then from there if you have any very low priority stuff you wanna put on another LVD that disconnects earlier then go for it.. but I cant see any need for more than 2 priority circuits.. critical loads (fridge/heater) and low priority loads (radio, fans, etc) if you want to stagger em, but thats far too complicated for most unless your leaving it unattended for a good while, I could see doing it in an off the grid cabin well before an expedition rig.

 

[Dr Gil]

Dreadlocks, you are correct about the ARB LVD. Like you said, I wanted to have a redundant LVD for other uses.

 

[dreadlocks]

if you want you can setup the ARB on the other side of the system LVD and set it up to something like 11.8v.. so it tries to go a bit longer.. and consider everything else as a secondary load..

I use the BMV-712 in tandem to controll the Battery Protect relay.. the Battery protect is set to cut off at my absolute minimum voltage regardless.. the BMV is set to disconnect a little before it and alarm.. so lets say BatteryProtect/LVD drops at 11.8v, the BMV disconnects at 12.0v.. I then wired a switch up in parallel with the BMV and labeled it reserve power..

With this setup you can drain the battery to 50% and system will disconnect all loads and alarm.. if its a situation where you really need that extra 20% of capacity you can flip the reserve switch and it will over-ride the BMV and let the LVD go down to 30% SOC before hitting the final failsafe to keep you from doing irreversible damage to your battery.