Out of curiosity:
I wouldn't have thought that the difference in output of the alternator is the issue here, unless your Enerdrive DC-DC allows more than 70A output.
That is, irrespective of whether you have a 70 or 100A alternator, the DC-DC charger will only output to the Aux battery whatever the max rating it says it does(eg. 25A or 40A, or whatever model you have)
Quick look at Enerdrive's products shows a 30A and a 40A.
So like Tim posted, a DBS that gives power directly via the alternator would be a better investment, if any changes are to be made to your setup.
Adding a 100A alternator over a 70A alternator still only gives you max current capacity of the DC-DC charger(ie. either only 30A or 40A).
And having a solar setup to charge whilst your driving along won't help either. Again, the DC-DC is limited to it's maximum capacity.
So if it's getting power via the alternator already(at it's maximum), then the solar array won't add any more to the Aux battery.
Only possible caveat with that is that it may be set so that it prefers solar power over DC power, so it could draw (eg. 10 Amps via the solar) and the other 20 or 30 amps could come from the alternator to keep it maxed out.
Don't know anything about the DC-DC charger you have, so can't be accurate with it's operation.
Maybe reply with the model of DC-DC charger you currently have.
On the topic of the 70A alternator and charging the lithiums whilst driving. technically, the alternator is not charging them at all .. the DC-DC charger is. So you already posted that it's lithium capable, so whatever the lithiums batteries are capable of absorbing is what they will get from the DC-DC charger. SO the answer here is yes .. the 70A alternator for that situation is fine.
Read the fine print on the lithium batteries, they can't all be charged very quickly. They have internal controllers for both charge and discharge.
eg. it could be that these cheap sounding lithium batteries can only be charged at (say) 10A max or something. They may also be limited to a maximum output allowance too.
eg. lets say that they only allow 20A output maximum, but you run some accessories(lights, fridge whatever) that exceeds 20A at few times. What happens to the electronics inside the lithium batteries. They usually reset, but if cheap and nasty enough could burn out or something.
So as an example(not knowing what Li batteries you are referring too), there are currently two different Voltax branded Li batteries on offer on on ebay for approx the money you mentioned.
One is 'deep cycle'.
The "Deep Cycle" version is about $200 more(give or take). Note the specs. This particular model is listed as Lithium Iron. The cheaper(by $200) version is LiFePo4.
The major difference in what I refer to above is the output capability(discharge). The cheaper one has a 50A max draw, the 'Deep Cycle' Iron model does 100A.
The other point of difference is the maximum (continuous) charge. The Iron one allows 40A(ie. input), and the LiFePo4 is limited to 20A input.
ie. if you wanted the cheaper (~$500) Lithium batteries, they only allow 20A input. So if you had a 40A DC-DC charger and a 100A alternator, AND a 200w+(ie. about 15(ish)A ) solar array .. the LifePo4 batteries will only allow themselves to charge at 20A anyhow.
I'm pretty sure my words are accurate here .. maybe the more technically adept members like Tim will correct any info I've contributed. Noting that my numbers are purely the theoretical listed by the devices referred too. In real world the actual numbers will be slightly different.
My post is to alert the OP that there is no point in going for a larger alternator or larger array of panels, if the batteries they want, can't make use of that extra power output by any mods made.
Be aware of the limits of the stuff you intend to purchase, before you figure out doesn't deliver the required capability.
Arthur.
'99 D1 300Tdi Auto
'03 D2 Td5 Auto
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