Trouble searching.... DC to DC charger for larger battery banks

[DiploStrat]

The usual rule of thumb is that the charging rate for lead acid should be 20% of the battery capacity in Ampere Hours. So, with a 100Ah battery bank, you would want a charge rate of 20A. Lifeline, on the other hand, calls for up to 5x the battery size. Thus a 500A charger for a 100Ah battery. (I have had Lifeline batteries on both of my trucks.)

Charging is a balancing act. In order to charge a battery, the charge source has to be at a higher voltage than the battery. With a lead acid battery you want to provide as many amps as the battery will accept without boiling over. With an open lead acid, AKA flooded lead acid, you can top up the liquid. With a valve regulated battery, which includes AGM, you cannot. Thus there is a general tendency to specify a more conservative charge rate to avoid the loss of moisture. One reason that temperature compensation is important.

As soon as a lead acid battery starts to charge, it builds up a surface charge and this raises the internal voltage and has the effect of reducing the charge rate. So even with a big charger, the rate tends to drop to around 50A pretty quickly. Smaller battery to battery chargers, like the REDARC, compensate a bit by offering slightly hotter (i.e. higher voltages) in their various profiles. Sterling Power offers bigger chargers, more profiles, and, finally, the option of a completely custom profile. (At least on some units.)

A battery to battery charger can compensate for small voltage drops (circuit resistance) but you will notice that all of them call for fat wires and recommend erring on the size of larger wires.

Modern US pickups (with 200A to 500A alternators) are quite capable of 100A charge rates for longer than a 400Ah battery bank will accept it. The thing you have to check is the voltage, if your alternator peaks at 13.9v your amps won't matter that much.

The key thing is that the charge voltage reach the appropriate range, typically 14.4v for AGM and even higher for modern FLA. If your alternator cannot provide this, you will need it from somewhere else, typically solar or shore power.

And even with a perfect Battery to Battery charger (DC-DC) you may not ever drive long enough to compete the charge. Lifeline, and, I suspect others, warns that the greatest danger to their batteries is that they are not charged long enough. This leads to partial charge and sulfating which reduces battery life.

GEEKS read these:

Rolls battery manual: https://rollsbattery.com/wp-content/uploads/2018/01/Rolls_Battery_Manual.pdf

Lifeline battery tech manual: http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf

 

[john61ct]

The usual rule of thumb is that the charging rate for lead acid should be 20% of the battery capacity in Ampere Hours.

true for FLA.

Lower is better for GEL I believe

> Lifeline, on the other hand, calls for up to 5x the battery size.
Quoting max CAR is silly. .4C is good enough, 40A per 100AH.

Yes Sterling is great for DCDC

> The key thing is that the charge voltage reach the appropriate range, typically 14.4v for AGM and even higher for modern FLA. If your alternator cannot provide this, you will need it from somewhere else, typically solar or shore power

Whole point of DCDC, use the alt if you got it.

> And even with a perfect Battery to Battery charger (DC-DC) you may not ever drive long enough to compete the charge.

As long as 3-4 days a week, 7 hours is enough. Or big amps early AM, finish the long tail with solar.

Or LFP, whole PSOC / long tail issue is gone completely.

At high cost.

 

[javajoe79]

Great info and thanks for your help everyone. To add missing info... I’m planning on space for at least 4, 6 volt 225ah golf cart batteries and at least 200w solar. Alternator is a Prestolite idlepro 190a. Capable of 60% of max amps at 650rpm. Voltage I don’t know. I’ll measure it next time I run it. I won't have much load. Engel fridge, minimal lighting, espar D5, a blower fan or two and maybe an auxiliary coolant circulating pump

Any reason I shouldn’t run a dc dc charger? I’m considering the redarc 40a. It has selectable charge profiles for different batteries.

 

[john61ct]

Waste if you don't need it, throttles current of alt output.

Put a 50%SoC batt on it, track voltage vv rpm and time.

If not within spec for batt, then buy the DCDC

 

[javajoe79]

I see what you're saying now. It would take much longer to charge the bank with the dcdc charger than without. So when I am doing the math, I should use half my Ah right as I wouldn't want to drain the batteries past 50%. Doing that with a 450ah bank discharged 50% I get a charge time of 2.4hrs at idle with 114A. Would it then make sense to bypass the dcdc charger at first then use it to top off the bank at the proper voltage?

 

[DiploStrat]

I see what you're saying now.

Sorry, but you don't. At least not yet. This is not hard, but it can be complex.

It would take much longer to charge the bank with the dcdc charger than without.

Depends on the charging voltage of your vehicle. If it is high enough and your cables short/big enough, then maybe, because your alternator can probably provide more amps. But if your vehicle voltage is too low, then you can drive all day with a relay system and accomplish nothing. That is why DC-DC chargers were invented; to assure a high enough charging voltage, and, by extension, a faster charge.

So when I am doing the math, I should use half my Ah right as I wouldn't want to drain the batteries past 50%. Doing that with a 450ah bank discharged 50% I get a charge time of 2.4hrs at idle with 114A. {/QUOTE]

Bad math and not likely in the real world. Charging a battery is not like filling a tank with water; it is a chemical reaction and follows its own rules. The first rule is that a lead acid battery quickly builds a surface charge and this greatly slows the charging process. (Charge rate is determined by the difference between the voltages of the battery and the charge source, the greater the difference, the faster the charge. Hence the demand for a higher voltage to assure that charging actually continues.) Second rule is that you need about one hour of absorb charging for each 100Ah of battery. The aborb stage is required to allow the surface charge to dissipate. So real world numbers might be:

-- 400Ah battery bank down 200Ah overnight. (That is a lot, more than I use and I cook electrically.)

-- So we need to recoup 200Ah. Guess about 75A for the first 30 minutes, 50A for the next 30 minutes, then 25A for the next 30 minutes, 10A for the next, ... you get the idea. Then, when you reach about 90% of charge, you shift to absorb for about four hours.

Would it then make sense to bypass the dcdc charger at first then use it to top off the bank at the proper voltage?

Actually, the CTEK SmartPass/D250S combo does exactly that. But there may be better/smaller/less expensive options.

As noted repeatedly, you need to know the sustained, steady state voltage of your vehicle's charging system. If it is over 14v, then a relay system is probably best. If it is 13.9v, then you either need to raise the voltage (there are options) or use a DC-DC charger. The latter may be easier.

More on charging a battery here: https://cookfb.files.wordpress.com/2016/09/battery-charge-slides.pdf

(N.B. Not all of the statements in this post are technically correct, they are intended to be informative and communicative.)

 

[john61ct]

I should use half my Ah right as I wouldn't want to drain the batteries past 50%.

Not a hard rule, just try to average that, only occasionally go much lower if you need to

> charge time of 2.4hrs at idle with 114A

No, with lead, CAR drops quickly as SoC climbs

> It would take much longer to charge the bank with the dcdc charger than without.

100% Full as per endAmps takes at least 5-7 hours anyway, even with 500A, so let's just talk about getting to 80-90%.

Time for that depends on the bank CAR, amps available, and voltage level.

If alt voltage is too low measured at the bank, you need the DCDC anyway.

If V is to spec for your batt, you don't.

 

[javajoe79]

Thanks for this. Definitely learning here and it's not helping me to randomly google info either. It's all over the place....

Just checked the idle voltage and it's 14.2. I will do the half drained test like mentioned above and post back about that.

 

[javajoe79]

See I got myself mixed up. I know the stuff about absorption rate going down as soc goes up. I just forgot.

So to avoid having to idle for hours on end at times, seems like I need more solar than 200W and most likely as you said, the dcdc charger isn't needed. I'll just do a relay setup or even a manual switch plus a solar controller.

 

[john61ct]

Depends on the bank.

14.2 is too low for FLA and many quality AGM, unless temps get hot. DCDC should give temp compensation.

Depends how closely to spec you want to coddle the bank for max longevity, can mean the difference between EoL (70% SoH) at 2-3 years, or shooting for 6-7.

For GEL may be just right.

Too high for my LFP, need Sterling's BB for that.

 

[john61ct]

If you use ICE charging in the early AM, up to the SoC point where CAR is around what the panels are putting out,

or at least where the solar is enough to get to 100% as per endAmps by late PM,

then that's "enough" solar.

Can always increase panels later, unlike the bank.

Doing so reduces your ICE hours per week, a good investment.

 

[john61ct]

Only current produced by the panels above what's needed to carry your loads is available for charging.

But then as SoC rises and CAR goes down, you have "free excess" output to put into "opportunity loads".

 

[DiploStrat]

Slightly different spin.

14.2v is probably fine - the exact voltage will depend on temperature, state of charge of the battery, etc.

The rule of thumb for solar is 100w per 100Ah of battery, but more is always better. If you add solar, and you should if you can, then make sure that your solar controller can absolutely hit the voltages specified by your battery manufacturer. This is the same idea as what CETEK are doing with their relay/B2B hybrid - you get the advantage of lots of amps when the target battery is low and then the higher voltage during the absorb stage when it is most useful.

 

[john61ct]

Namely CTEK D250S Duo + Smartpass

Good kit, but the solar side not so great.

Also like Redarc not adjustable

 

[DiploStrat]

To be precise, the REDARC is not infinitely adjustable, but it does have four profiles which match most batteries that you are likely to use.

Note that all run a bit hot to help compensate for lower (25A or 40A) output.

The CTEK does not offer different profiles.

https://cdn.shopify.com/s/files/1/2...ut_Instruction_Manual.pdf?7204406381448510702