Charging Voltage Question

ToddS · Aug 2, 2019

I'm FAR from an expert... and others on here will have more informed answers for you surely. However, I will say that for me at least, it is totally normally for voltage to read low while the system is under load... mostly higher-draw things like pump motors... but even a cell phone charger or a handful of LED lights will drop it by 1/10 volt. A pump drops it by 1/2 volt or so. If the fridge just kicked on, and a pump was running, and a cell phone was charging, and your propane solenoid was on (etc. etc.) I could easily imagine it (mine at least) displaying 0.8v lower than the battery actually had.

jssailem · Aug 2, 2019

If you were running a load on the batteries at the time you looked It could have caused your read to be low. You want to have the batteries at a resting state after a couple of hours to understand the charge. You also might do an electrolyte status check to get a better state of charge value.

Or you can get a meter. The new Balmar SG200 meter is just the thing. Or if you willing to calibrate the meter the Vectron meter will also give you a good idea of your usage and SOC.

Check out MaineSails website (Marine How To) for detail on the elusive SOC of your batteries and how to keep informed.

Terry Cox · Aug 2, 2019

ToddS said:
However, I will say that for me at least, it is totally normally for voltage to read low while the system is under load... mostly higher-draw things like pump motors... but even a cell phone charger or a handful of LED lights will drop it by 1/10 volt.

Ditto, here. Our boat has two 8D AGM batteries. Readings might be a bit different than yours, but doing what you did, our house bank will read lower when both reefer units on, but will recover when they shut off. On our second day on the hook, with no draw, voltage will drop to about 12.3 - 5. When both reefers kick on, that will drop to about 12.2 - 3, then recover to the 12.3 - 5 when they cycle off. Our Link-1000 provides better information, such as amps consumed. Our house bank is rated at about 500ah. When consumption rises above -130 / -150ah, voltage is usually in the low range of 12.2 - 3. That is when I run the Genset for an hour and half to load the house bank to -20 ah, which is good for another two days of normal use. That has worked for our cruising life style for several years, most of which we spend on the hook.

Davidasailor26 · Aug 2, 2019

Monterey385 said:
Just a quick update, Genset running charger, 4 hours still at 30 amp max, voltage now at 14.3 on charger. So depending on draw (which should be no more than 3-5 amps) I’ve put between 100-120 amps into the batteries I think. Curious to see at what point it will start to drop amps going in.

Keep in mind that as soon as you hit 14.3 volts (assuming that's your bulk voltage) the amperage will start to come down from the 30A max. Once it gets acceptably close to 0 the charger will go to float. No really good way to know what amperage is going in during that phase, without a shunt.

Terry Cox · Aug 2, 2019

Monterey385 said:
So here is the odd part ... the minute I started the charger (ProNautic 12-30P), the charger read 12.9 and after 2.5 hrs at 30 amps she is reading 13.9 volts (still taking 30 amp bulk).

Odd. In our situation discussed above our Xantrex three stage charger will deliver a bulk charging voltage of 14.2 - 4 for most of the hour and a half. When it reaches an accept charge, it will drop to 13.6 - 8 and remain there even when it reaches a float charge. Normally, a float charge is only reached when we have shore power connected for two to four hours, but even then the charge voltage will remain in the 13.5 - 7 range.

Maine Sail · Aug 2, 2019

Monterey385 said:
Just a quick update, Genset running charger, 4 hours still at 30 amp max, voltage now at 14.3 on charger. So depending on draw (which should be no more than 3-5 amps) I’ve put between 100-120 amps into the batteries I think. Curious to see at what point it will start to drop amps going in.

When bulk ends..... If your charger is set for 14.4V current will begin to taper once 14.4V is attained. If 14.6V current will begin to taper then. You battery was discharged to about 50% SOC so you're going to need to stuff a lot of energy back into it. Keep in mind that once you hit absorption you still have a long way to go to get to 100% SoC, like many more hours....

Blitz · Aug 2, 2019

The start-up of the charger immediately took the load off the batteries, thus they recovered to 12.9. The amps were just passing through the batteries to the loads on at this time.

BTW - the ProNautic 12-30P is a little undersized for your 420 amp hour house bank.

It may help you if you set up your charger on a custom charging profile. I assume you set up you charger just as a flooded battery bank, and are not using a custom charging profile. A custom charging profile will let you set the battery manufacturers recommended absorption and float voltages. (you should also do an equalization charge once in a while)

A Victron battery monitor which measures amp in and out would benefit you in understanding your amp hour draw and state of charge. I'm thinking you might have been drawn down more than you think.

rgranger · Aug 2, 2019

Monterey385 said:
Is it possible that my batteries really weren’t at 12.1 and/or because they were under load it was an abnormally low reading?

I like the analogy of pressure in a water pipe when I think about voltage and loads. Your battery is analogous to a big water bladder tank that holds electrons in your wire (pipe) at a certain voltage (pressure). When you put on a load (open the spigot) the electrons (water) escapes the system and lowers the voltage (pressure). The battery (bladder) works to replace those electrons (water) to keep the voltage (pressure) stable.

The voltage drop you see under load will be proportional to the current draw and the ability of your battery to deliver charge. If the drop is significant, you may want larger sized wire (pipe) coming from your battery to your systems.

Blitz · Aug 2, 2019

Monterey385 said:
is there a rule of thumb for charger size to total bank hours? I would prefer not to have to run my charger this much on the mooring.

Going off of memory but I think Maine Sail once indicted somewhere that he likes to see 10-15% of bank size (drop the units)
300 amp-hour bank - 30 amp charger
420 amp-hour bank - 40 amp charger.

May be different for different battery types due to acceptance of the charge of different battery types.

Also, probably your charger manual would have some guidance.

dlochner · Aug 2, 2019

Monterey385 said:
is there a rule of thumb for charger size to total bank hours? I would prefer not to have to run my charger this much on the mooring.

Yes, 10% of the amp hour capacity of the battery plus some extra to cover loads while charging.

For a 420 ah bank, a 50 amp charger is a good choice.

jssailem · Aug 2, 2019

dlochner said:
For a 420 ah bank, a 50 amp charger is a good choice.

Great choice. Smaller charger more time needed to return bank to 100% SOC. 40 amp charger will do it but needs more time. (Time = charger works harder). The 10% model helps the charger to work less. Electronics do not like to work any harder than necessary, kinda like my daughter. Too hard too hot = complaints..

Hello Below · Aug 2, 2019

Monterey385 said:
is there a rule of thumb for charger size to total bank hours?

My recently purchased Trojan T125s came with a Users Guide that states in part:
Maximum Charge Current* 13% of C20.
The * (asterisk) states:
If charging time is limited contact Trojan Technical Support.
I would assume each battery manufacturer has these kind of specs available.

dlochner said:
plus some extra to cover loads while charging.

Dave, what happens to the extra charger output when the loads are removed such as a fridge? Potentially could this not cause the battery’s max charge current to be exceed?

dlochner · Aug 2, 2019

Hello Below said:
Dave, what happens to the extra charger output when the loads are removed such as a fridge? Potentially could this not cause the battery’s max charge current to be exceed?

Modern chargers are smart chargers. They sense the battery's condition and charge accordingly. Ohm's law governs this. The charger will provide the current and voltage the battery can accept.

When a load is added, such as a refrigerator, the charger will increase the current it puts out up to the charger's limit. Let's say the batteries are accepting an absorption charge (constant voltage charge) and the charger is putting out 20 amps. When the refrigerator kicks on, the charger will increase the current it produces to meet the new demand, however the battery will only accept a certain level of charge. The increased output of the charger will squeal the battery's acceptance plus the refrigerator's demand. Does that make sense?

The max charge rate is determined by the battery, not the charger. The charger has limits to how much it can produce, but not how much the battery can accept. If the battery bank is a 200ah bank and can only accept a 20 amp charge and that is all the charger will put out even if the charger is capable of putting out more than 20 amps.

Benny17441 · Aug 2, 2019

That is a reason why Pure voltage readings are unreliable, a battery right after charge will show a higher voltage than actual and a battery just under load will show a lower voltage. Allowing the batteries to rest for 2-3 hours might not be a practical option. To use a volt meter to calculate state of charge requires the making of an educated guess based on consumption data and observation. There is one other thing a weak and sulfated battery with reduced storage capacity can show voltage at 12.7V indicating is 100% charged, but put a load on it and will rapidly decline. You can get an automotive load tester for around $35, they work in a 12V and 6V scale. They will give you a voltage reading and when you flip the Test Switch it will impose a load on the battery. Test each battery individually and a weak battery will usually dip in voltage much lower than the rest. A single weak battery can bring down a whole bank. Once you establish a baseline you will quickly detect weak batteries. I know what my readings were for a brand new broken in battery and I know right now that my whole house bank is down to about 90% in 3 years. I will keep them until they hit around 80% and then replace.

Hello Below · Aug 2, 2019

dlochner said:
The charger will provide the current and voltage the battery can accept.

I completely understand with all that you have clearly written. Thank you. I am not disagreeing. But I am also trying to understand what is meant when the Trojan User Guide suggests that the maximum charge current be limited to 13% of C20. This, as written, I take as the user is to limit the charger output to that spec (ie size the charger to 13% of C20) rather than the battery only accepting that input. They also state if charging time is limited contact them for technical support. This leads me to believe that a battery, under certain conditions, will accept greater than 13% of C20 if the charger output is greater than that. So I guess the question is: Can a charger ever be sized too large or will the battery simply accept what Ohm’s law allows?

Maine Sail · Aug 3, 2019

dlochner said:
If the battery bank is a 200ah bank and can only accept a 20 amp charge and that is all the charger will put out even if the charger is capable of putting out more than 20 amps.

The charger is only limited by the battery once the “limit voltage” or absorption voltage has been attained. Below that point, this is called bulk charge or constant current, the battery will accept what ever amperage the charge source can throw its way.

If you have a lot of current, the bulk or “constant current” stage won’t last very long and the battery will the become voltage limited. Once the battery is voltage limited is when it decides how much current it will accept (function of voltage, current and SoC) so as to not overshoot the voltage limit..

Low Charge Current = Long Bulk Charge
High Charge Current = Short Bulk Charge

This article illustrates this: How Fast Can an AGM Battery Be Charged

When considering generator charging the two bullets below summarize the difference between a .2C charge rate and .4C from a 50% DoD. As can be seen with an identical 1 hour charge duration the .4C charge rate put in an extra 14% of energy over the identical duration...

50% SOC @ .2C For 1 Hour = 71% SOC – Remained in Bulk

50% SOC @ .4C For 1 Hour = 85% SOC – Exited Bulk @ 19 Minutes

Trojan Batteries can certainly accept much more than .13C in charge current. Trojan's guidelines are really for golf course type charging and .13C is a good charge rate to go by if you have many hours to charge.

If you are limited in the amount of time you want to spend charging you can "fast charge" Trojan batteries at upwards of .25C to even .3C. Trojan may recommend .2C if they don't know what you have for charge equipment, temp compensation etc...

There seems to be some confusion around charger sizing and shore side use sizing is different from generator use.

Generator Charging - BIG CHARGER
Shore Charging Only - 10% to 20% of Ah capacity or a charge rate = to .1C to .2C

JimInPB · Aug 3, 2019

Monterey385 said:
...
Is it possible that my batteries really weren’t at 12.1 and/or because they were under load it was an abnormally low reading?...

If the wires on the volt meter do not go directly to the battery, then other devices on the same wires will effectively form a resistor network due to resistances in the wires & the meter will read a percentage of the voltage that is actually present at the battery.

Unless the volt meter wires go directly to the battery, with no other loads drawn from those wires, you will read a voltage that is lower than it should be when the battery is under load.

When charging, the opposite can be true, if the charge wires are intermingled with the meter wires.

I normally make a strong effort to run meter wires directly to the battery, or at least the battery switch.