spacer spacer spacer spacer

agm and wet cell what to do.

Post ReplyPost New Topic
< Prev Topic Next Topic >
Copper - Posts: 66
Copper spacespace
Joined: October 11, 2012
Location: Kentucky, United States
Posted: September 14, 2014 at 9:49 PM / IP Logged  
Hello everyone! I have a question. I am about to install a Audiopipe 1800.1D in my Mountaineer along with a Soundqubed HDS312-D2. I will be running at 1ohm. My alternator went out on me so my alternator is new and its a 130a alt. I went ahead and purchased a new battery as well. I got a Autocraft Platinum AGM with 120RC. My old battery is only a 1 1/2 years old, it is a Duralast Gold. I am going to run 1/0 pure copper wire on my alt, engine ground, chassis ground, and as power and ground to the amp. My question is this. Both batteries are in perfect shape should I use them both? If so how? Or should I be fine without the Duralast Gold? I will never play it without the engine running unless it is down low. I have heard mixed opinions about mixing AGM's with typical wet cells however it is what I see in most peoples cars. I am totally lost and would appreciate any help =)
When in doubt....kick it!
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 15, 2014 at 6:36 PM / IP Logged  
Batteries that are connected in series or permanently paralleled need to be matched. That includes model, batch, age, history, temperature, & connections.
Batteries that are paralleled need only be matched by type & voltage (eg lead acid) and be isolated from each other when not being charged or not required for the load (eg, paralleled for cranking, winching, audio etc).
Hence you can parallel your batteries provided you use a battery isolator. Have a look at charge-light controlled relays (eg google "oldspark uibi" else voltage controlled aka smart battery isolators.
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 15, 2014 at 7:27 PM / IP Logged  
PS - there are many "opinions" re the mixing of batteries, but my first sentence above sums the situation.
Batteries in parallel is the one situation where batteries can be mismatched wrt age & capacity etc provided they are of the same type like lead-acid (wet, AGM, gel) and voltage (eg, 6-cell aka 12V) noting that other types like NiMH & Lithium have limitiations wrt charging requirements etc.
However lead-acid charging suits charging in parallel and since the charging voltage is higher than a full battery's voltage no battery can discharge into another.
A vehicle's charging system is voltage based - ie, a constant voltage system (even if it may switch from a charge voltage of 14.2V - 14.4V down to a "float" voltage of ~13.4V). As per circuit theory, powering devices from separate identical ideal supplies is the same as powering those devices from the same ideal supply, hence multiple batteries can be charged from the same alternator.
Batteries should not be left in permanent paralleled connection because they interact... The lower voltage battery will discharge the other(s). (A good battery will not hold up a bad battery - the bad will pull down the good!)
In some cases that can be hazardous - especially when AGMs are involved - eg, heat, acid vapors, fires, explosion.
Hence why parallel batteries should be isolated when not in use.
FYI - the reliability of 2 identical paralleled batteries is HALF that of the single battery (in simple terms, ie, the failure rate of the BOTH of them doubles).
How soon such isolation needs to occur is situation dependent.
The above should address & counter many of the arguments that I have seen over the eons.
Tho some arguments have merit, they usually involve failures, and why would you knowingly connect a faulty battery to a system? (Such checks and maintenance are a different issue.)
And most arguments essentially say that batteries can NEVER be paralleled - at least not in our typical systems where batteries may be separated (boot/trunk & engine bay) or differe4nt types (wet & AGM) or capacity (80AH cranker & 7AH for PC) - and IMO there are MANY with such systems that have operated safely and successfully for decades.
And it is assumed the system is robust enough - ie, the alternator is appropriately sized.
Weak alternators that blow due to short-term overloads - eg, after jump starts - are another issue.
In most vehicles, alternators will have enough capacity to charge several batteries - even if the initial charge current is much higher than the max alternator output,
Recently I read how deep cycles could not be paralleled with crankers because the deep cycles would boil dry long before the crankers fully charged. I'd like to hear the author's logic... they both charge from (say) 14.4V...
Member - Posts: 3
Member spacespace
Joined: September 24, 2014
Location: Oregon, United States
Posted: September 24, 2014 at 11:43 PM / IP Logged  
^^ is a very helpful post. I just registered here to ask a few similar questions. I'm not trying to hijack this thread or anything, it is related to the topic at hand.
Basically, I am planning on adding a 35ah AGM in the trunk, isolated from the starter battery via relay. I chose this method for both cost and space concerns.
Technically I could simply wire it up and forget about it. But I'd rather install a simple charge controller for the AGM. I don't need it to charge fast, in fact I want to avoid that. I won't be utilizing the AGM reserve power all that often, and when I do, I can simply disconnect the battery and recharge it on a bench.
What I do want, is to be able to discharge it completely, and not have to worry about charge current to the agm or both batteries competing for the alternator's love. Which is why I decided to put a controller between the AGM and alternator. I'm not sure if this is necessary or not, and I'm not sure if the AGM is capable of drawing any more than it's max initial charge current, but just seems the ideal thing to do for my application.
A potential problem I'm seeing with this, is whether or not my junk in the trunk, will be drawing from the AGM, instead of the alternator while the engine is running. If this were the case, then the charge controller wouldn't be able to keep up with the drain. I would assume the alternator would provide ample power, while the engine is running, but what's to say it won't draw from the AGM, or starter battery for that matter while the engine is running? The (higher) alternator voltage?
Member - Posts: 3
Member spacespace
Joined: September 24, 2014
Location: Oregon, United States
Posted: September 25, 2014 at 2:30 AM / IP Logged  
I drew up a basic diagram to clarify.
agm and wet cell what to do. -- posted image.
One diode is used to block the AGM from the starter/alternator while the isolator is closed (connected), and forcing any charge to go through the controller. The other diode is to prevent the AGM from attempting to charge itself through the controller while the isolator is open (disconnected).
Still I'm wondering what law determines that the load isn't drawing from the AGM while the alternator is functioning. Some people seem to think the alternator directly provides power to the electrical system while running. If this is the case, I should have no problems. Others think that the electrical system draws from the battery, and that the alternator keeps it charged in the meantime. If this is the case, the controller isn't going to provide a sufficient charge considering it regulates the charge to 4 amps max, while the load draws a good 10-12 continuous.
I will add that in the past, operating the load with a 10a charger connected to the healthy starter battery, would repeatedly trip the breaker of the charger, so obviously the bulk of the current was coming from the charger, and not the battery. I assume this is the case when the alternator is functioning, and that it is because the alternator is providing a higher voltage than the battery. Is this correct?
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 25, 2014 at 5:23 AM / IP Logged  
Well you are hijacking tho I don't think we nor OP vinchinzo mind much. It should be a new thread.
But whilst here...
Don't worry about the alternator. Unless it's one of the poor alternators I sometimes refer to, it will be fine.
Nor would I worry about any charge controller unless it's a buck/boost type in which case it's probably cheaper buying new AGMs - I doubt a charge controller is warranted. (You're in the USA so you shouldn't be like my fellow Aussies that are sucked in by somewhat amazing 2nd battery assertions and expen$ive products.)
Yes, as you suggest, the HIGHER alternator voltage means the AGM will not discharge. IE - since you will have "reasonable" wiring to your AGM, it will not supply the load whilst the alternator is charging. Reasonable means no more than fractions of a Volt voltage drop between the front alternator/battery and the AGM. And it assumes your alternator is charging acceptably. (FYI - typical main battery voltages during charging should be ~14.2 to 14.4V which means at least a 1.5V drop before a full AGM (12.7V) will start to discharge.)
See also my last block of ramble below.
Discharging "completely". For best AGM life you want to recharge it ASAP after any discharge, tho discharges of only 10-20% may not matter that much.
Forget the diodes. Without the charge controller you have a typical isolated dual-battery circuit - ie, front/main battery +12V (thru fuse) to isolator relay to (fuse to) 2nd battery +12V.
When NOT charging, to stop the AGM discharging you turn off the load OR have a battery protector (ie, low voltage disconnect) between the AGM and the load.
Do not excessively discharge AGMs. Unlike wet cells that can be recovered (somewhat...), AGMs will not recover.
A 12A will trip a 10A charger's breaker. Even without the battery recharge current, the extra 2A should trip the breaker.
FYI - at 10A, a good 40AH battery should last about 3 hours (that's to 1.80V per cell => 10.8V across the terminals) after which it should be recharge immediately.
Otherwise the general rule is to NOT discharge crankers by more than 20% and deep cycles by more than 50%. That's for "reasonable" battery life. (Some deep cycles claim 80% discharge but limiting to 50% typically means 3 times the battery life.)
20% discharge of a 40AH at 10A means less than 1 hour of discharge.
For the isolator you may want to use a simple (and superior) charge-light controlled relay. (EG - google "oldspark uibi".) But they may require a buffer (transistor or MOSFET, with RC delay for newer DP type or ECU controlled alternators) if the alternator's voltage regulator's L or D+ (charge light) circuit cannot supply the current to drive the relay (or intermediate small relay).
FYI - I used a UIBI with a 38AH UPS AGM (ie, high recharge & discharge currents) as a 2nd battery which had a max observed recharge current of ~45A (when perhaps 10-20% discharged) which blew my initial 30A fuses. I replaced the fuses with 50A self resetting circuit breakers and thereafter had no problems. I used a 60A relay with a coil current of ~250mA or less off various typical 1980s-1990s Jap alternators with integral regulators.)
People that reckon the battery supplies the load and the alternator charges the battery have little understanding of electrics - especially automotive electrics.
The battery and alternator and loads are all in parallel (ignoring voltage drops thru cables & fuses etc) and hence when the alternator voltage exceeds the battery voltage, the alternator supplies ALL loads (including battery recharge). IE - the highest voltage supplies or overrides the others.   
The battery is only used when the alternator is insufficient for demand - ie, when cranking ('cos the alternator isn't then charging anyhow, even if it could supply the >>100A cranking current at less than idle speed), or during initial load changes (like brakes, flashers, fans, wipers, headlights etc before the alternator compensates), or when demand exceeds alternator output - eg, wipers and headlights when idling; big sound systems (at low else any RPM), or bad batteries (high recharge currents) or bad alternators (reduced output current).
Member - Posts: 3
Member spacespace
Joined: September 24, 2014
Location: Oregon, United States
Posted: September 25, 2014 at 7:07 AM / IP Logged  
Thanks for the detailed response. I figure that a charge controller assures that the AGM is kept fully charged, or at least close to it, as the car alternator will not accomplish that goal. And at the same time, provide a controlled bottleneck between batteries, so that the AGM when deeply discharged isn't exceeding it's max charge current, which is about 10 amps. Although the 20 or so meters of 4 gauge wire, coupled with the fuses at the battery terminals, may prevent this anyway..
I've read reports of people not having any issues with a conventional relay isolated setup, so I will probably take that route to begin with and see how it goes. It simplifies the setup, which is already complex enough as it is, with multiple systems that need to coexist and interact with each other. I plan on fully charging each battery every couple of weeks anyhow. Again, just don't want to damage the AGM, or the starter battery for that matter, while simply driving.
I do plan on deeply discharging the AGM, that's the reason I'm adding it to the system. The entire system shuts down at 11v automatically, not to mention I have a V/A meter up front to monitor the voltage and power consumption. Again I will install the AGM with quick disconnects so that I can haul it inside and recharge it ASAP after deep discharges.
That leads back to my concern about the max initial current. If I have discharged the battery to say 11v, and then power up the isolator without starting the engine and it's regulated alternator, what kind of current will be rushing from the fully charged starter battery, to the fully depleted AGM, through two 50a fuses and 25m of 4 gauge copper stranded wire?
Am I being obsessive?agm and wet cell what to do. -- posted image.
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 25, 2014 at 8:52 AM / IP Logged  
Obsessive? No - not yet. I consider it caution, and relaying what you have heard. Alas I deal with many ridiculous comments by many (NOT you - but other "forums" etc) but then there are comments which may technically be (pedantically) correct but have little effect in practice - eg, limiting charge current.
I'm wondering why you say your alternator will not fully charge an AGM? Your alternator should be set to 14.2 - 14.4V (not the olde traditional 13.8V). That should be enough to fully charge an AGM since they are still a lead-acid battery.
However I am aware of f.ex Optima that claim higher charging voltages - noting their "unlimited current" PROVIDED the battery is below whatever temperature (~50C?) which it won't always be in many engine bays and boots (trunks).
And some newer vehicles have higher than 14.4V charging (eg 14.9V) for reasons I don't yet know. Maybe that's my lack of awareness of recent (chemical) changes etc, but I have also yet to see that 14.2-14.4V does NOT fully charge such batteries.
(That reminds me of Australia where for years certain entities make $hundreds to $thousands per customer based on the "voltage drop" between the alternator and (remote) battery. Now even if there is a large voltage drop due to initial charge current, as the battery charges and the current drops, so does the voltage drop so the remote battery should eventually see close to the alternator voltage. And voltages like 13.4 or 13.8V will fully charge a battery, it'll just take longer, and - for voltages below ~14.0V - it won't be enough to reverse sulfation which occurs in "not full" batteries, hence reducing battery life. (That increase from 13.8V to 14.4V along with the introduction of Calcium etc are IMO the main reasons batteries now last 6-10 years instead of 3-5 years or less.)   
Charge current limiting...
Every battery has some limitation on recharge current. For (wet) lead acids it's typically ~10% of its AH C10 rating. For AGMs it may typically be 20% of the C10 AH rating.   
Hence a 40AH wet cell should be limited to 4A and a 40AH AGM limited to 8A.
As I said, my 38AH AGM would typically take up to 40-45A hence exceeding its 20% of 38AH = ~8A limit fivefold! However that quickly reduced to less than 10A within one minute.
Every OEM vehicle has(?) more than enough spare alternator capacity to provide the battery with whatever current it will accept, yet no manufacturer has (traditionally...) bothered with current limiting.
And AFAIK battery warranty claims have never countered with claims of OEM vehicle overcurrents.
Of course such initial overcurrents are beneficial for battery maintenance - ie, blowing away sulfation after discharges - especially for wet cells. (Such "maintenance" is somewhat more detailed for AGMs, but yet again initial overcurrents do not seem to be a major issue.)
FYI - above 14.4V "long term" is a no-no because batteries will begin to gas significantly at such voltages. Hence wet cells boil & dry out, and AGMs suffer immediate damage because there is no free liquid to boil off (and cool the plates) and the AGMs will heat significantly.
And FYI - it's the internal resistance of a battery (which increases with state of discharge; and age) along with its internal voltage (state of charge) and external applied (charging) voltage that determines its charge current, ie [ V-alternator minus V-internal ] divided by its internal resistance R (Ohm's Law: V=IR hence I = V/R).
And as a rule of thumb, an AGM's internal resistance is roughly half the internal resistance of an equivalent (type & AH) wet-cell battery. Hence why AGMs typically have twice the short circuit current of wet cells, and why they'll typically absorb twice the charge current of a similarly discharged wet-cell. Luckily AGMs typically have double the allowable max recharge current.
Anyhow, I wouldn't worry about limiting charging current unless your AGM is especially sensitive to it (eg, maybe a deep cycle?).
OR unless a suitable charge conditioner is cheap compared to a replacement AGM.
I'd suck it & see and maybe consider a charge regulator/conditioner if the first AGM prematurely fails for no other apparent reason.
And a charge conditioner is yet another component to fail...
I know many that have OEM wet cell crankers with AGM secondaries and use plain relays for isolation (whether charge-light controlled or voltage sensing aka "smart" battery isolators) and have no problems.
The only problem I am aware of are permanently paralleled battery setups (eg, thermal runaway if AGMs are involved; or that batteries are statistically replaced 4 times as often in dual-battery systems because the "double fail rate" means BOTH batteries fail twice as often UNLESS the batteries are monitored at sufficient intervals).
Such systems vary from small AGMs (from 1.2AH) for PC supplies to hundreds of AH of AGMs for audio and winching systems, or camping and solar systems (usually deep cycle).
You might see some examples on which I recently left.
Of course some claim that batteries must be of similar AH rating, or of same type (wet or AGM), or even "matched", but that does not relate to isolated systems which are only paralleled when being charged (or when required for winching or extra capacity).
Again mp3car if not the12volt should provide ample evidence of the above (tho most date back a few years).
However I left mp3car because of one fool that insisted that batteries need to be matched, yet he seemed to accept single-pole isolation (ie, +12V only, not the GND) and non-colocated batteries (eg, one in the engine bay and one in the trunk) without symmetrical connections (eg, equal +12V to +12V and GND to GND cabling with the +12V supply taken from one and the GND taken from the other)... IOW any "matched" battery will not remain matched for long...
Luckily that fool demonstrated how clever he was to others too - like using a resistor in series with relay coils (of similar resistance to the coil) to limit their spikes (I wonder how many failures or fires that might account for?), and some Arduino output via a COMMON GROUNDED opto coupler etc when just a MOSFET (and 2 resistors) is far superior. Not that I like to hang poo on twits that fail to answer questions of clarify and substantiate their claims... agm and wet cell what to do. -- posted image.
As to (high) discharge currents from one battery into another... [ Oh boy, another mp3car epic comes to mind - a few years ago some "diode isolator" nut used that and other arguments. Of course he contradicted his own statements and a certain Old Fart successively defeated all his claims. ]
Firstly such currents will not be that high - at least not as high as being charged from an alternator since the max voltage a battery can be is 12.7V. That is only a concern with failed batteries - ie those with collapsed cells. And hence why batteries should not remain parallel connected (when not being charged) - ESPECIALLY AGMs as they can go into a thermal runaway mode and eventually (cause) flame or explode.
Secondly, that NEVER happens with automated isolators such as the UIBI or voltage controlled isolators since they only connect when the alternator is charging, hence one battery cannot discharge into another.
There are some exceptions, eg, alternators that might extinguish their charge light during cranking, but then you are cranking and both batteries will be discharging into the starter motor.
Also if the UIBI or smart/voltage-controlled isolator (relay) is manually overridden.
In all cases it may be that the interlink protection operates - hence the advantage of self-resetting circuit breakers. (EG - 50A and smaller self-reseters cost under $10. I had one alternator that "pre-extinguished" its charge light thereby interconnecting my 2 batteries during cranking. However I was not aware of any breaker trip - probably because of the short cranking times tho my 140A reduction starter (instead of OEM 240A starter) also helped. My breakers were 50A; my relay 60A; and my cable was much heavier than required.)
FWIW, a simple basic calc:
Fully charged (12.7V) 40AH wet cell with 20mR (R = Ohm) internal resistance.
50% discharged (12.0V) 40AH AGM; lets assume 10mR internal resistance (half that of the wet cell) even though it will be far higher because it's discharged.     
I = V/R = (12.7 - 12.0)V/(20 + 10)mR = 0.7V/30mR = 23A. That's not too bad. Of course the AGM will probably have a resistance higher than the wet cell so it'll be less current - eg, even if only 20mR, .7/0.04 = 17.5A.
But if that's whilst cranking, BOTH batteries will be supplying the starter current.
And that requires a manual override if automated isolators are used.
Again, search "oldspark uibi" with optional or the12volt filters for more info and (manual override) circuits etc, and of course the piece de resistance - what the uibi acronym is (not that I like to hang poo...).
PS - the biggest concern is "weak" alternators. I have often written how I abandoned my short involvement with 1980s-1990s Bosch alternators because they blew their main diodes after jump starts or running bad batteries. I recently read that some GM alternators behaves similarly. Whilst I consider alternators that burn windings to be of poor design, IMO blowing the power diodes after a mere jump start is totally unacceptable!   
They might also not tolerate the added load of a second battery, nor bigger battery or changing to AGM for the same reasons.
(I have never had any major issues with my Japanese alternators - even after reverse connection to a battery!)

Sorry, you can NOT post a reply.
This topic is closed.

  Printable version Printable version Post ReplyPost New Topic
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot delete your posts in this forum
You cannot edit your posts in this forum
You cannot create polls in this forum
You cannot vote in polls in this forum

Follow Follow on Facebook
Saturday, December 10, 2022 • Copyright © 1999-2022, All Rights Reserved Privacy Policy & Use of Cookies
Disclaimer: *All information on this site ( ) is provided "as is" without any warranty of any kind, either expressed or implied, including but not limited to fitness for a particular use. Any user assumes the entire risk as to the accuracy and use of this information. Please verify all wire colors and diagrams before applying any information.

Secured by Sectigo spacer spacer spacer
Top spacer spacer