Amps, Ratings and Draw.

I have the 130 amp alternator.

I'm adding a DC - DC charger for a 100ah LifePo battery to power a camper's accessories.

I've seen recommended charge rates from 25a to 50a. For my use I was looking at 40a charger. It occurs to me I really don't know what my alternator can handle. I don't want to put a strain on my alternator or know what type of headroom I should be looking for.

 

If nobody answers I’d research posts and videos on this. Mav on YouTube put a DCDC charger on his F150 for aux battery and idk if he changed alternator.

there are bigger alternators but they cost a lot and may require other changes for it to work right. And hopefully not overcharge the regular battery.

my last secondary power unit I charged with solar and through cig lighter port that the plan is later do 400w anytime mod harness for more juice.

another thing to look at is thicker cables. Something like jeepcables. That may be required to run a bigger alternator.

I think there’s various charge controllers out there that might play a part

 

What does the battery say? Does it have a built in BMS?

Max charge on most of those are 0.5C, (50amps for 100AH battery). But that's max, not what's ideal.

I wouldn't go over 20amps, will be better for the battery. And you also won't need to run big ass 4-6g wires for 40amps of load .

 

I am a noob on this … wont the charger just take what the alt offers? Or, could turning on the charger cause a spike so as to blow out the alt fuse? Trying to learn this myself.

 

Speaking of the accessory battery & its charger as a separate subsystem, a 40A charger sounds very reasonable for a 100Ah lithium battery. It will not give the maximum possible safe charging rate but it will be a lot gentler to both the alternator and the battery being charged if it tops out at 40A rather than say, 75 or 100A. Does the DC-DC charger have configurable charging profiles?

As long as the DC-DC charger is reasonably "smart" it should be smart enough to not overload the alternator at any point in the charging cycle. If it is especially well-designed it might also have nice features like "soft-start", i.e. when it turns on it ramps up the current draw from 0 to (say) 40 amps over a period of a couple of seconds, rather than just instantly "cutting in" a 40A load on the alternator with no ramp-up.

What I'm less clear about is what the basic load is on a stock Tacoma with a flooded or AGM lead-acid starting battery, and how much 'headroom' there is above that to be safely dedicated to an auxiliary / accessory battery charger.

Consider posting the make/model of the DC-DC charger to allow TW forums peeps to research it & learn more.

 

As someone who has run multiple chargers for a house LFP the last couple years, I can say “I’m your Huckleberry.”

As others mentioned the first thing you need to check is your battery specs for max charge rate. Chances are for a 100A LFP it’s going to be less that 50A (0.5C). DC chargers with a lower rate will work fine but charge a little slower, however in general they also cost less. Determine for yourself how much each of those factors matter.

I currently run a

$539 REDARC BCDC1240D REDARC 40A in-Vehicle DC to DC Charger with MPPT — Dual Input for Solar & Alternator Charging, Green Power Priority - 12 or 24V Battery Charger Suitable with AGM, Gel, Lithium LiFePO4 Batteries

REDARC BCDC1240D which can charge at 40A and it works great on my 206Ah SOK battery. Zero problems with power output of the 130A alternator in this scenario because I don’t have a ton of other power-heavy accessories running at the same time. If you have more lights, stereo/subs, crazy power draws, or towing a trailer then maybe it will not be enough but for the vast majority of people it’s fine.

But long story short for only a 100Ah house battery just pick whichever one. I wouldn’t spend the coin on a 50A charger unless you plan to have bigger batteries later. Lots of options in the 20-30A range that will treat your wallet and battery just fine. Ain’t rocket surgery.

 

I have a X2Power AGM battery as the engine battery and the diode to change the output of the alternator to 14.6v. I also know by increasing the voltage the rated amperage will drop somewhat

I've also purchased a Dr. Prepare LiFePo4 100ah for a house battery. I liked it because it comes with a "Battery Charging Hub" that plugs into the top and I can use to draw up to 10a with a USB out, USB-C out, DC out, and cigarette lighter out. It also has a DC input. My needs will quickly exceed the output of this Hub but it is nice as a back-up/secondary.

I looked at the

$539 Redarc BCDC1240D REDARC 40A in-Vehicle DC to DC Charger with MPPT — Dual Input for Solar & Alternator Charging, Green Power Priority - 12 or 24V Battery Charger Suitable with AGM, Gel, Lithium LiFePO4 Batteries

Redarc BCDC1240D and it is a really nice unit. Pluses are it is dual input, I'm planning on adding 200w of solar on the roof and it would be a good way to get it into the system. I also like it is built robust enough it could be mounted in the engine bay. But yeah, that comes at a retail price $$$504, currently $442 on Amazon.

We are getting a little ahead of ourselves though. I do think I will be fine, but still I'd like to know as a base what demands my Taco uses in amp draw and what the maximum is so I can get a general idea on how taxing I will be on the stock 130a.

 

If you want to know stock demands, go buy an amp clamp meter at your local hardware store. Clamp around the battery cable, start your truck, and start turning on all accessories to see how much they draw. Realistically you're probably only using like 60-70A max but not even all the time. Again, dependant if you tow a trailer and how much that is consuming but you haven't provided us any of that info so you'll have to do your own research.

 

An update to this thread, I found some info in this link:

https://www.tacomaworld.com/threads...a-amp-draw-and-alternator-loads-tested.821385

@Roadkill69 seems to have a lot of knowledge and also has a build thread on his M1 Pop-Up Camper.

 

That measurement you describe will only be be accurate if the truck is turned off. When running most of the current is supplied by the alternator some will pass on to the battery and some will pass directly to the load. In most cases the alternator's output will power most of the load and that extra current will be seen on the wiring to that load but it won't all pass through the battery cables. Turn the motor off and now 100% is being fed from the battery and passing through it's cables so at that point the measurement you described would be accurate.