Learn Hardcore Indonesian: Biar saya mengisi ulang baterai senter sekarang. - Let me recharge the flashlight battery now.

More formal or stronger: Biarkan saya… or Izinkan saya… (“Allow/permit me…”).
Note: biar can also mean “so that/even if” in other contexts, but here it clearly means “let.”

Is Biar saya… polite? When would I use something else?

It’s polite in everyday speech and sounds helpful/volunteering. To sound extra courteous:

Add softeners: Biar saya saja…, Biar saya… ya.
Ask permission: Boleh saya…? / Bisakah saya…?
Formal: Izinkan saya mengisi ulang…

Can I say Biar saya isi ulang… without meng-?

Yes. Both are fine:

More formal/complete: Biar saya mengisi ulang baterai senter sekarang.
Natural/informal: Biar saya isi ulang baterai senter sekarang. In casual speech the base verb (here, isi ulang) after an explicit subject (saya) is very common.

What’s the best verb for “to charge” a battery: mengisi ulang, mengisi daya, or ngecas/mengecas?

All are used; choose by register:

Formal/tech: mengisi daya (baterai), mengisi ulang (baterai)
Neutral: isi baterai, isi ulang baterai
Colloquial: ngecas/mengecas (from “charge”): Biar saya ngecas baterainya.

Is mengisi baterai (without ulang/daya) acceptable?

Yes, in everyday speech mengisi/isi baterai is widely used and understood as “charge the battery.” For formal precision, prefer mengisi daya baterai or mengisi ulang baterai.

Why is it baterai senter and not senter baterai?

Indonesian noun compounds are head-first: baterai senter = “flashlight battery” (battery of a flashlight).
Senter baterai would suggest “a battery-powered flashlight,” which isn’t the intended meaning here. You can also say baterai untuk senter (“battery for a flashlight”) when speaking generally.

Should I add -nya to say “its battery,” like baterainya or baterai senternya?

Use -nya for a specific, known item:

General: baterai senter
Specific/definite: baterai senternya or baterainya (“the flashlight’s battery / its battery”)
Example: Biar saya isi ulang baterai senternya sekarang.

Can I say I’m charging the flashlight itself (not mentioning the battery)?

Yes, if the device has a built-in battery people often say:

Biar saya cas senternya.
Biar saya isi ulang senternya. If the cells are removable, baterai senter is more precise.

Where can I put sekarang? What about sekarang juga or dulu?

Placement: end or beginning are both natural.
- Biar saya isi ulang… sekarang.
- Sekarang biar saya isi ulang…
sekarang juga = “right now/immediately.”
dulu adds “first (before other things)” and is very common:
- Biar saya isi ulang baterai senter dulu.

How do I mark plural “batteries”? Do I need reduplication?

Indonesian usually leaves plural unmarked. Use numbers/quantifiers when needed:

dua baterai, beberapa baterai, semua baterai.
Reduplication (baterai-baterai) is grammatical but rare in everyday speech.

Should I use saya or aku?

saya = neutral/polite; safe everywhere.
aku = informal/intimate with friends/family.
So: Biar saya… (neutral) vs Biar aku… (casual).

What does yang add in Biar saya yang mengisi ulang…?

yang focuses the doer: “Let me be the one (who) recharges it.”
It’s a polite way to take over a task: Biar saya yang urus. You can add saja for “just me”: Biar saya saja yang…

How do I politely ask someone else to do it?

Neutral: Tolong isi ulang baterai senter sekarang.
Softer: Bisa tolong ngecas baterai senternya sekarang?
Formal: Mohon mengisi daya baterai senter saat ini.

How do I talk about time/aspect (am doing, just did, will do)?

In progress: Saya sedang mengisi ulang baterai senter. (I’m recharging it.)
Just did: Saya baru (saja) mengisi ulang baterai senter.
Will/plan: Saya akan/mau mengisi ulang baterai senter nanti.

Is senter the same as “torch” (UK)? What about lampu senter?

Yes, senter = flashlight/torch (UK). Lampu senter is also heard and understood but is somewhat redundant; senter alone is most common.

Does biar also mean “even if/although”? Could that confuse things?

Yes, in other contexts: Biar hujan, saya tetap berangkat. (“Even if it rains…”).
In your sentence, biar clearly means “let” because it’s followed by a volunteer subject (saya) and an action.

Why not mengisikan ulang? When do I need -kan?

Use bare mengisi ulang for “recharge (it).” The -kan often marks a beneficiary/causative:

Imperative for someone else: Tolong isikan ulang baterai ini. For your own action, (meng)isi ulang is standard.

Any quick spelling/pronunciation tips?

baterai is the standard spelling (you’ll hear colloquial batere/batrai).
senter = SEN-ter.
mengisi = məng-EE-see; ng is a velar nasal.
Colloquial ngecas is common; formal writing prefers mengisi daya/mengisi ulang.