battery banks

I just carry an extra (charged) set of batteries in the truck, then either jumper cable them over in parallel with the batteries on the trailer (through a set of fused cables), or swap them out when the trailer mounted ones get low. Then I'm not paying the weight penalty of the extra battery bank unless I'm dry camping for a long time. I'm using the Costco 2GC golf cart batteries.

Lifeline says that their AGMs are fully charged when the charge rate drops to 0.5% of the 20hr rate. That is 500ma for a 100AH battery.
After that, float charging is optional because the battery is ready for use.

The charge Volts are 14.3 +/- temp comp and the float Volts are 13.3 +/- temp comp.

Does your charger have a battery temperature sensor(BTS) cable?

Lifeline has a handy "Table 5.1 Charging Voltage at Different Temperatures for a 12 Volt Battery".

What is your definition of full charge?

Lifeline also gives a formula for estimating time to full charge in case you don't have any Amp measuring metering:

Time to Reach Full Charge = {(DOD/100) x Rated Capacity (Ah) ÷ Rated Output of Charger (Amp)} + 2 hours.

For example, charging a 100Ah battery at 50% DOD

with a 25A charger would take:
{(50/100) x 100 ÷ 25} + 2 = 4 hours to reach full charge.

If a 10A charger is used, it would take:
{(50/100) x 100 ÷ 10} + 2 = 7 hours to reach full charge.

At DOD below 50%, a minimunm of 20A is required, otherwise a float charge procedure is needed. Don't try to charge 4x100AH at 75% DOD with less than a 80A, 14.3V Absorption charger.

If you substitute unlimited(~) as the charger rate in the formula you get 2hrs because you would instantly start Absorption, or 0+2=2.

In battery charger terminology the 14.3V translates to Absorption in the sequence Bulk-Absorption-Float.

WFCO does NOT have a proper Absorption due to the 13.6V spec.

What does your charger do?

HTH;
John

i.e. 300 amp-hours vs 100 amp-hours on wfco will both take 168 hours to fully charge.

Sam Spade wrote:

pianotuna wrote:

If it is a lousy converter (wfco) it will take exactly the same time. About 168 hours.

Exactly the same as what exactlty ?

I allow that you might be right but I just don't understand.

Seems to me like the only time this would be true is when the charging source is able to supply WAY more current than a single battery will draw when dead. I don't think that would be true with a 40 amp source.

Sam Spade wrote:

westend wrote:
Few batteries can take even 40 amps of charge current beyond the first few minutes.

OK.....so how many individual batteries will take less than 20 amps when their state of charge is really low ? Because that is the point at which you could charge two at the same rate as one.

The question is unanswerable.

Lets say you have four batteries, for example. All of them have been discharged to 75% SOC. You connect a 40 amp charger to them and the 40 amps of current is shared by all four, only up to the point that any of them will no longer accept their apportioned allowance of current. That point may be in a few minutes. The rest of the charge time, there is much less current going to each battery as they have gone beyond that point where 40 amps can be accepted. If an experimenter was looking for a difference in the whole time it takes to charge one battery vs charging four, it would be a few minutes as the bulk of the charging time is between 90% SOC and 100% SOC, where total current is not important.

Sam Spade wrote:

pianotuna wrote:

If it is a lousy converter (wfco) it will take exactly the same time. About 168 hours.

Exactly the same as what exactlty ?

I allow that you might be right but I just don't understand.

Seems to me like the only time this would be true is when the charging source is able to supply WAY more current than a single battery will draw when dead. I don't think that would be true with a 40 amp source.

Allow me to clarify.

The WFCO is well known to charge at 13.6V and not at the claimed 14.4V.

Now, when you apply unlimited Amps to the battery, the battery's Voltage will immediately rise to 13.6V. When this happens, current momentarily stops flowing. Soon the battery will absorb the current and the battery's Voltage will drop allowing current flow again. That is absorption.

If we ignore battery temperature effects, the battery will take 168 hours to charge due to the 13.6V.

Of course we should not ignore battery temperature because that is the limiting factor for current acceptance.

An adjustable charger with a BTS feature will provide an optimal charging current by adjusting Voltage so as to maintain battery temperature within specs at all times.

FYI, Optima AGM batteries have a fast charge spec of unlimited current at 15.6V so long as the battery temperature does not exceed 125F. That is not an optimal charge rate but does give one some idea of what the Optima can accept.

I have been 100% AGM since 2009. Three of my four chargers have BTS; the fourth is a WFCO.

HTH;
John

westend wrote:
Few batteries can take even 40 amps of charge current beyond the first few minutes.

OK.....so how many individual batteries will take less than 20 amps when their state of charge is really low ? Because that is the point at which you could charge two at the same rate as one.

Sam,
It is all about battery acceptance rate. Few batteries can take even 40 amps of charge current beyond the first few minutes. As the batteries accept charge, the current diminishes.

Also, we shouldn't be charging "dead" batteries. I'm guessing you used that term to identify a battery that is low in State Of Charge. No foul.

pianotuna wrote:

If it is a lousy converter (wfco) it will take exactly the same time. About 168 hours.

Exactly the same as what exactlty ?

I allow that you might be right but I just don't understand.

Seems to me like the only time this would be true is when the charging source is able to supply WAY more current than a single battery will draw when dead. I don't think that would be true with a 40 amp source.

Hi,

The smallest is about 40 amps. If it is a decent converter (Iota, PD, or a few others) charging time may be identical for 300 amps-hours vs 100 amp-hours. That is not "a correspondingly longer time to recharge"

If it is a lousy converter (wfco) it will take exactly the same time. About 168 hours.

Sam Spade wrote:

pianotuna wrote:

No, they don't take a lot longer to charge.

It depends......mostly upon exactly HOW they are being recharged.

I am guessing here a bit but how many small travel trailers that come only with a single battery do you know of that come with a 50 amp charger/converter ? My semi-educated guess is somewhere near to none.

Anyway, I didn't say "a lot" longer. But it could be a lot depending on circumstances.

I have three smaller AGM's in one bank, located driver's side under kitchen counter top cabinet just in front of the axles. I have another bank of 2 X 6V FLA's located on the passenger's side in a vented battery box I built, also just in front of the axle wheel well. I wanted the batteries inside to keep tongue weight down and to afford a good temperature when camping in cool weather. It also offers some security above the typical tongue mounted box.

Vented box:




AGM bank:


AGM's under this shelf--


Behind a cabinet door (the one with gauges)--