While battery chargers are rated as 100, 125, etc amps, battery chargers do not control number of amps flowing into batteries. Batteries control amps being accepted, which reduces as battery 'fills-up' because of higher resistance in a charged battery. Battery charger ratings are the maximum amps capable.
For our large battery amp-hour sizes, battery bank capacity setting is mainly used to estimate when to switch from Absorption to Float. Example: when the charger sees Absorption amps less than 8 amps for 3 minutes, switch to Float. Larger batteries switch to Float at a higher amp setting.
Batteries often limit charge amps to no more than 25% of the bank's total amp-hours. Example: two 8D's are approx 450 amp-hours, so a battery charger rated at over 112 amps could be de-rated.
Chargers have voltage settings for Bulk, Absorption, and Float. This can be a custom setting or we can choose from one of the many profile choices built-in to charger software. Settings are matched to battery type.
Battery's maximum & optimum voltages decrease when battery is warmer, and increase when battery is cooler, the reason for the charger's temp probe. Charging at too high a voltage (no matter what the battery size) can cause electrolyte to heat causing expansion, and for Gel & AGM, lead to damaging exhaust through emergency pressure relief valves. Chargers have a temperature coefficient setting that changes maximum voltage based on temperature changes.
At any given temperature, Gel battery types usually charge at a lower voltage than AGM or flooded, so Gels can take longer to charge. The higher the acceptable charge voltage, the shorter the time needed to recharge. It is a fine line that keeps battery charging at the highest acceptable voltage for the battery's temperature.
Modern 'smart' chargers usually have three (or more) charging steps, with the most common being Bulk, Absorption & Float. The idea is to get the battery charged safely in the fastest time. Each step has several parameters with multiple voltage, amp & time settings.
BULK charging does not manage amps, but does monitor battery voltage and when it rises to some setting, Bulk changes to Absorption. Bulk starts at the maximum amp capacity of the charger, and often very quickly, the battery reduces its amp acceptance.
ABSORPTION charges at the voltage setting, and monitors charge amps being accepted by battery and at some setting Absorption changes to Float.
FLOAT charges at a reduced voltage and monitors for battery voltage drop and time elapse, and may change back to BULK and start the cycle over.
Multiple simultaneous charging sources, like alternator, 110v-charger can 'fool' smart-chargers. Example: If a low-amp capacity charger (like solar) is putting a few amps into an 'empty' battery at 14.2 volts, a 100-amp charger may think the battery is ready to be switched from Bulk to Absorption, when it would be better served to stay longer in Bulk.
Thank You Barry.
Your post should be framed in Gold and should be put at the head of the Forum Technical Section as well as be placed at the head of the "Beam Alarm" sections on Alternators, Chargers and Electrical systems.
I am patient with newbies and non-electrical types. wanting to understand our electrical systems, especially if they have no electrical background or experience.
I become impatient and it becomes tiresome trying to communicate with those that profess to be experts and should know better (the ones that consistently and repeatedly steer people wrong regarding our elegantly simple diode isolators, diode voltage drops and diode reverse current leakage.
Properly sized/rated diode isolators, exactly in accordance with the OEM FT design, work reliably, without human intervention or intelligence, and will last forever, if one doesn't abuse them with failed batteries.
I have no fault with Wyatt's design change which eliminates dual battery banks in favor of a single bank. Wyatt monitors the SOC of each of his three 8D batteries and uses that bank for all 12Vdc needs, with solar charging supplement. Nor do I have any problem with Brett's design change which goes to robust, marine, manual, dual battery bank, manual switches. BUT, both of these require that the humans involved KNOW what they should monitor and (without fail) operate the battery charging manual controls. Unfortunately, many of us suffer from CRS, and the KISS design (like the OEM isolator) requires less human interaction/less corrective maintenance/less budget, in the long run.
I have helped several different coach owners (all but two were SOB's) resolve charging issues, most of them stemming from "Smart, zero voltage-drop", VSR's (Voltage Sensitive Relays). The VSR's were of many different brands and the only ones that I haven't seen fail were Blue Sea Systems ACR's (Automatic Charging Relays).
I have no idea why so many otherwise helpful people get their knicker's in a twist, about a diode's or isolator's operating characteristics: It is all basic science and common sense, just like if you drop a bowling ball off the top of a ten story building; It goes down, not up!
- They are obsessively focused on an Isolator's loss of 0.6 Vdc to 0.9 Vdc (manufacturer and load dependent) across the diode, when supplying current to a battery. So what? That's how a PN junction, power diode works, JUST LIKE THE OTHER (7) TO (12) POWER DIODES, that the rectified DC current has already passed through, inside the alternator, in arriving as DC Voltage and Current at the battery isolator. The diodes all dropped the same amount of voltage in rectifying the dc voltage and current. They all got warm (or hot) from the current passing through them. That's why the isolators have cooling fins.
- They are obsessively focused on isolator center terminal voltage, when the alternator is at rest. Power Diodes (and all PN junctions) have a tiny amount of reverse current leakage. If an isolator does not have B+ from the alternator applied, it will always have some small voltage present due to reverse current leakage. Means nothing. Just basic electronics science.
- "Smart" VSR's, unlike diode isolator's, are difficult to troubleshoot. In actual operation, they are difficult to understand for an expert, much less a layman. They do a myriad of confusing things when they partially or totally fail, or even, as Barry has pointed out, during normal operation, when voltage sources compete.
- The sensing circuitry in "Smart" VSR's is delicate, is easily "zapped" and is often unreliable in comparison to the much more robust Diode Isolators.
- As Barry has pointed out, many (Most?) "Smart" VSR's do so many things that they generally fail to operate optimally because of competing voltage sources or the humans (that operated the device adjustments) doing so outside of the design intent of the device. Easy to do with a "Smart" VSR, impossible to do with the OEM Isolator.
- Higher Current, Mega Budget Alternators may fill a narrow window of needs (dry camping, deep discharge of your house battery bank and then hit the road without generator help.....neophyte error), but encouraging neophytes to exceed OEM parameters is generally uncalled for. Trying to push more amps into a battery bank (with fixed OEM design resistance characteristics), in less time, without increased temperature compensation sophistication, is just asking for budget breaking heartbreaks.
- An alternator needs to know what the start battery (B+) voltage is. That is why the alternator sense wire needs to go to the start battery B+ terminal (gets more complicated and manual in Wyatt and Brett's cases). Common sense should tell anyone that voltage sensed anywhere else in the coach is unpredictable and irrelevant in properly controlling an alternator's output.
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Enough of my views, and thank you again, Barry, for your calm explanation of how our OEM charging system works.
HTH,
Neal
Plus one for the blue sea automatic charging relay. Finally fixed the engine battery charging issues especially with our 14 watt solar panels in dry storage.
+1 on the Sticky Thingy. Both are great pieces.
And on behalf of us Newbies and general ignoramuses, I appreciate the above lesson. I kinda know this stuff but it's nice to read it stated differently. ^.^d
Neal, Thanks for your input on charging. Hard to argue with simplicity, durability & longevity when it comes to isolators and start battery bank charging devices.
Years ago, we dipped our toe into using voltage-sensing relays (VSR) as a replacement for our trusty Trik-L-Start diode, to keep start batteries charged. After many months, we found that VSR devices just did not charge as well as our original diode devices in all situations. We spent a lot of time trying to make them work, even buying a different brand VSR device, but gave up trying to recoup our costs.
We did experience an isolator failure several months ago, and decided to replace it with the same style isolator.
(Cole Hersee 48160 Battery Isolator from Amazon) A few months ago, I wrote up our VSR experience:
Float Charing/maintaining (http://www.foreforums.com/index.php?topic=33088.msg299741#msg299741)
Battery Isolators and Automatic Charging Relays - Blue Sea Systems (https://www.bluesea.com/resources/58/Battery_Isolators_and_Automatic_Charging_Relays)
More good info.
Barry, great, clear explanation of battery charger operation.
However, I would like to add two items:
1. The change in charge voltage based on temperature changes is so dramatic that NOT using a temperature probe is foolish because it will result in damage to hot batteries and/or gross undercharging of cold batteries.
2. The reason that the charge voltages for a Gel battery are lower than for other batteries is because the Gel battery has lower internal resistance resulting in higher charge currents (NOT because the Gel battery is fragile). This means that charging a Gel battery is more efficient than charging a flooded or AGM battery.
Neal you said:
"The diodes all dropped the same amount of voltage in rectifying the dc voltage and current."
Technology advances have made available "very low drop" diodes which drop the voltage much less than the OEM diode isolators which Foretravel installed in our coaches.
I do agree with you that the voltage drop does not matter from a practical point of view.
I also experimented with "Smart" VSR's (before adopting a single battery bank), and came to the same conclusion as you did (they do not work well).
I also agree that a larger alternator (than the OEM 160amp) is of little value to most motorhome owners because I power my refrigerator from the inverter (powered by the 160 amp alternator) when driving. I can than comply with the law by turning the propane off at the tank when driving.
My guru buddy mentioned that trying to temp control the alternators output to correctly charge the start batteries did not work due to their location in the engine compartment where the road heat and engine heat signaled the alternator to reduce the voltage way too low.
Maybe why Foretravel equipped every unicoach with optima batteries? If a battery is hot then our use is overcharging them by quite a bit?
My buddy mentioned the charge voltage dropping to nearly 12 volt.
Quite likely that the spiral wound optima can dissipate the overcharging heat best. Judging by multiple 10 year plus reported service life posts here seems to work.
Maybe why Foretravel mounted then in a low, sideways to the airflow during a drive location?
We all are surely overcharging the start battery in hot weather.
Not sure how an ilsolator would affect the hot battery overcharging versus an auto combiner?
Wyatt's mention of the gels lower charging voltage due to less resistance is the same reason lifelines AGM's have a different .2 volt lower charging voltage program from a magnum charger versus the other AGM's and charge 20% faster
Wyatt, your point 1 is really important as we see a 1-volt difference between cold and hot charge voltages. In fact we made our own custom temperature coefficient so Xantrex 2.0 charge voltages would track the battery manufacturer's recommended charge voltages from 40 (14.2-volts) to 120 (13.2-volts) degrees. Assume it would take years off a batteries life if charged a 14-volts all summer long. Maybe that is why our batteries last over 10 years.
What got me in trouble with VSR's was during a hot summer, our temp compensated maximum house gel battery charge voltage was below the VSR's closing voltage and our start batteries were never see any charge. Sure with the diode volt drop on our Trik-L-Start, the engine battery would not be seeing a high voltage, but it would be enough to keep up.
It drove me crazy that when I used the correct charge voltage, my fixed voltage Blue Sea would not connect, so I then purchased the Magnum combiner where I could vary the closing voltage, but it ended up more complex and not 'automatic' for year around use, so the two combiners are resting with other parts.
I was not aware of Gel's lower resistance, as a reason for lower charge voltages. So they may not take longer than other batteries to get 'full' because of the lower voltages.
We are aware of Bob's comment, that it is easy for alternator to have too high voltage for start & house batteries in warm weather.
When we last rebuilt our alternator, we had a variable voltage regulator installed and crank it down toward a lower end setting, which could take longer for batteries to be charged.
We figure that the alternator only has to replenish start current, because we generally drive off with pretty full house batteries. And since we don't want the alternator to pump, via isolator, too high voltage into our house batteries on a long drive, the lower alternator voltage setting works quite well for our situation.
Since alternators are not aware of battery temps, it seems better to favor a lower end voltage.
Interesting idea Bob on the spiral red-top batteries having more external cooling surfaces...
But I don't think that Foretravel took into consideration the physical location to mount our start batteries in a cooler place.
Although we are quite fortunate to have our start battery bank in a very accessible location in our coach layout, just look at the newer coaches with batteries tucked up near engine and exhaust in an all but impossible location to maintain.
So what is a good compromise voltage at the start batteries when being fed from the alternator? 13.6? 13.8?
I plan to turn down my alternator voltage as it currently feeds 14.1 at the start batteries and outside posts of the battery isolator. Center alternator post of the battery isolator is at 15 volts, diodes drop it at the outer posts.
I need to find and read the posts about setting the alternator voltage. This will wait for better weather as currently it's cool weather and raining. I just want to turn it down a bit before summer high temps.
Dan, Our alternator can have three different types of voltage regulators:
1) Fixed at one voltage,
2) 3-step that uses an internal bridge between three small posts to choose one of three voltages,
3) Variable (probably less popular) that uses an external facing small potentiometer shaft that has a small turning adjustment.
For alternator setting, I would say 13.4 which is the mid-chart OPTIMUM Gel float voltage for 70 degrees. 13.4 also is also the MAXIMUM float voltage for 100 degrees.
What do others recommend?
Four years ago, when I installed new AGM batteries and solar, I also replaced the diode isolator with a Blue Sea ACR. The system has worked flawlessly, and I no longer need a battery maintainer to keep the engine batteries charged.
I did the same, and set my alternator output to 13.6 volts. I have solar also and while the ACR may cause problems when charging with multiple charging sources, I do monitor both of my battery banks, and have not seen a problem yet.
Wolfe10 mentioned using the same alternator output voltage we use. 13.6 volts. Engine computer sees 13.1 to 13.3 according to VPMS..
Our two solar panels put out 270 Watts(14 amps) and are tied into the coach at the back of the refer.
That happens to be a voltage drop of about one volt ending up with 13.6 at the house batteries. Worked out.
That output turns out by luck to be enough to activate the auto connect feature of the Sea Land combiner. Worked out again.
As long as both banks have top condition batteries in them with good cabling and maintenance every thing seems to work well.
Went to the coach and turned on the 12 volt door switch and plugged my two digital volt meters into the dash just to see everything's condition. Have been sitting for 30 days or so.
13.5-13.6. Same every time I check it.
Engine running shows 13.4 to 13.5 at the same gauges on the dash.
Yes the solar is still on. Feeding the system. Shows the same digital readouts at night so I have not factored the solar into anything other than the mpg as with everything fully charged I understand the alternator should reduce its amps to zero?
Less accessory hp draws=more mpg. Tiny difference but what the heck.
Running the inverter on gen or shore shows the temp voltage variations that the magnum with its BTMS adjusts.
Wyatt's system is as perfect as can be done as all of his bank charging is temp controlled.
No easy backup his way of course. No mistakes.
Everything is a compromise. How to lessen them is a good goal.
So 13.6 from the alternator. 13.6 from the solar. 14 amps from the solar turns out to be enough to operate the auto combiner and over time recharge discharge house batteries.
The magnums input when operating shows how far off from perfection I really am.
Close. No cigar.
30 years ago I put the oem chargers on timers and put rudimentary solar on my coaches which was my limit on technology then.
Plus I replaced every battery in every used coach I ever sold. Had to. They were normally damaged in use.